Achilles' Heel



""You must punch him in the center of his right buttcheek.""

- Doctor McNinja, The Adventures of Dr. McNinja

Any seemingly Nigh Invulnerable character (particularly The Dragon) will inevitably have some key weakness that can and will be exploited.

In any show where the main character is superpowered in some way, an Achilles Heel is essentially mandated; otherwise, they would never be able to get into any kind of personal peril to drive a story. For example, the Gemini Man's 15-minute limit on invisibility (any more and he dies), or The Greatest American Hero's utter ignorance of the full capabilities of his supersuit.

Aliens and monsters that are Immune to Bullets will usually have a fairly Mundane Solution Achilles Heel that only surfaces/gets discovered completely by accident when things look the bleakest. The archetypal example is the Martians from HG Wells' War of the Worlds, who shrugged off everything the armies of the world could muster, and then died from the common cold.

Other versions and variations of the Achilles Heel include:
 * Attack Its Weak Point: For video games, this is the spot the player has to hit.
 * David Versus Goliath: Not taking a seemingly trivial threat seriously.
 * Drama Preserving Handicap: A method to give some sort of weakness to a character who normally doesn't have one.
 * Fantastic Fragility: An in-built weakness to boost power.
 * Fatal Flaw: A psychological weakness, not a physical vulnerability.
 * Kryptonite Factor: A weakness to something that has little or no effect on normal characters.
 * Self Destruct Mechanism: Just in case....
 * Tailor Made Prison: A prison designed to contain a specific character, often accounting for their weakness.
 * Weaksauce Weakness: Arbitrary weakness to something common and mundane.
 * Why Did It Have to Be Snakes: A character-specific phobia.

Anime

 * Played With for the homunculi in Fullmetal Alchemist. Their immortality is fueled by the Philosopher's Stones in their bodies; Mustang tore Lust's out of her body attempting a one-shot kill, only for her to form a new body from the Stone. Turns out their only weakness is being killed a few thousand times (or the destruction of the Stone itself).
 * Pride, however, does have something of an Achilles heel in the form of pure darkness: since he uses shadow-manipulation powers, he's powerless in total darkness, because you need light to cast a shadow.
 * Alphonse is bound to his armor by his blood seal, and should that be destroyed, he's a goner.
 * Wrath's Ultimate Eye basically turns him into the equivalent of a Perfect Play AI: he can predict his opponents' moves with perfect accuracy and therefore wins almost every fight he's in effortlessly.
 * Played straight for the homunculi in the first anime, where they are pretty much immortal, but can be killed if one gets anywhere near the remains of the human body they were created from. Wrath averts this, though, but only because his remains were used as the raw materials in the attempt to resurrect him, so they don't really exist anymore to serve as a weakness.
 * The Zentraedi and Protodeviln of Macross and Macross 7 are both weakened by music - the Zentraedi because they haven't been exposed to "culture" for so long, and the Protodeviln because Basara possesses the Anima Spiritua.
 * Virtually any character in Get Backers have fantastical abilities but with very heavy handicaps, more often than not forcing the characters to pull near-fatal trump card moves. The best example is one character of the main duo, Mido Ban, who can subject people to dreams with his Jagan (Evil Eye) for exactly one minute in reality, but indefinitely in the dream itself, making it an ultimate twink move. The catch? He can only use it once on a person and three times total a day. Needless to say, a lot of antagonists find ways to avoid this or force him to use his Jagan needlessly until he's helpless.
 * Death Note's Shinigami, being grim reapers, are practically immortal... with two notable exceptions. First, a Shinigami's lifespan is extended by writing names in a Death Note: if a Shinigami forgets to write names, they will actually run out of time and die. Second is the fact that a Shinigami who intentionally uses their Death Note to protect a human - extending someone's lifespan rather than cutting it short - will die instantly. There are also a number of rules they have to follow while in the human world if they don't want to get on the bad side of the Shinigami King. Also, Ryuk is addicted to apples, and goes through some awkward withdrawal syndromes if he goes for too long without eating any.
 * That said, Ryuk's withdrawal can't kill him, nor can starvation, as he doesn't really need to eat. It's even possible that his withdrawals are psychosomatic.
 * In Science Ninja Team Gatchaman, the team's main vehicle, The God Phoenix, has a rather poorly thought out weakness in that all the team's sub-vehicles have to be docked for the God Phoenix's weapons systems to work. While that kind of weakness would make sense to create the plane's Fiery Phoenix effect since it's an exotic weapon system, to have it affect the regular weapons seems too much. Even worse, Galactor learns about this weakness and the visual clues for it and wastes no time to take advantage of it.
 * Although pretty much almost completely Immune to Bullets, the cyborg assassins of Gunslinger Girl are pretty much as vulnerable as any little girl when they are not focused. Furthermore, shooting them in the should be enough to kill them.
 * Many times on Saint Seiya, more than one character suffers from this. Shiryu's main means of attacking leaves his heart wide open to attacks for a fraction of a second, something a few people notice over the course of the story. Krishna likewise was only defeated when Shiryu struck his hidden "Star Points", and the Pope of Sanctuary's astral projection attacks left his body wide open to attack.
 * The Pope's one is not as bad as the others, as he's smart enough to do it on targets that are far away from him and without the psychic powers you'd need to counterattack while staying in a well-guarded location. Then Shun revealed his chain would attack without missing if he had even a vague idea of who he had to attack, and the Astral Projection actually allowed the chain to hit him. Then the Pope refrained from attempting it again in case Seiya, Shiryu or Hyouga pulled something like that.
 * Probably one of the most gag-inducing Achilles Heels in modern media, the Big Bad of Digimon Adventure 02's only weakness was the Power of Dreams.
 * In one of the earliest episodes of Inuyasha, a human artist gains the power to make his art come to life, and starts creating some of the most horrible creatures imaginable as an army to obliterate his enemies. However, once Inuyasha destroys the artist's inkwell -- which actually turns out to be a magic inkwell -- he's down and out.
 * Inuyasha himself has his own weakness: On the night of a new moon, he becomes fully human and loses all of his demon powers.
 * Subverted in Legend of Lemnear. The Big Bad transforms himself into a 50 foot demon cyclops, and the male hero: The warrior of Bronze declares
 * Goku of Dragonball survives a bullet and an axe to the head (relatively) unharmed, but if you grab his tail, he'll pretty much pass out. Eventually he trains himself to not be so affected by it. It even works when he's transformed into a giant ape.
 * Goku and Piccolo themselves exploit this weakness in the battle with Raditz. After that attempt failed, Goku didn't bother going for the weakness again, because he remembered that Saiyans could also manually detach the tail from the body. Attempted against Nappa and Vegeta, although Piccolo finds out rather brutally that the Elites aren't stupid enough to not train that weakness.
 * Goku also has a tendency of losing strength if he feels hungry, which results in his defeat in his first battle with Yamcha as well as his fight with Jackie Chun.
 * Makoto Shishio from Rurouni Kenshin is a very powerful swordsman, but can't fight for more than 15 minutes straight. If he does, he will overheat to death.
 * Nagi Springfield of Mahou Sensei Negima is described as invincible with no weaknesses. Except for the fact that he's an idiot who needs a cheatsheet to remember spells. He also has trouble pronouncing latin.
 * Alternatively, his commerade Eishun is a more serious character who for his strong focus on his training means a lack of human contact, and therefore a difficulty with women. This seems a common trait in Shinmeiryuu swordsman if Setsuna is any indication. Tsukuyomi takes this weakness too far in the other direction.
 * All Devil Fruit users in One Piece have the common fatal weakness of becoming completely immobile and useless in deep water. Other fruits have more specific weaknesses; for example, Luffy's Gum Gum fruit makes him immune to blunt force, and can only be damaged by being slashed or pierced. On the other side of the coin, Buggy the Clown's fruit makes him immune to being cut, so his weakness is blunt force.
 * In particular, the Logia Fruits, easily the most powerful of all the Devil Fruits, all seem to have a single weakness that negates their great power or their perfect defense. For example; Sir Crocodile, eater of the Sand Sand fruit, becomes solid when wet. And God Enel, eater of the Rumble Rumble fruit, which gives him lighting powers strong enough to destroy the entirety of Skypeia, is made completely helpless when pitted against Luffy, who is made of rubber and doesn't conduct electricity.
 * Logia Fruit users can also have their power partially negated along with their inability to be physically harmed when up against someone with a powerful Haki. See chapter 559 for all the gory details (Smoker vs. Hancock)
 * The Yato of Gintama have immense physical strength, but are sensitive to the sun and carry around umbrellas (which double as guns, shields (bullet-proof?), and are very durable, used something like swords) to protect themselves during the day. On one occasion a character died from being exposed to the sun only briefly, after years of being underground.
 * Zanpaku-tou in in Bleach all have some kind of a weak point. Hanatarou's can take the wounds it's healed and turn into an attack powerful enough to kill a Gillian, but afterwards it's a useless three-inch-long scalpel. Consider most enemies in Bleach have a horrible rape radius far, far greater than three inches, this makes him near useless in combat. Komamura's bankai can take down most enemies with just one hit, but any damage inflicted upon it also appears on Komamura, so if the enemy survived that hit and was able to retaliate... ouch.
 * Ukitake is prevented from getting involved too in battle by his Incurable Cough of Death. One wonders what he'd be like in good health, because he's fought Yamamoto and Starrk as it is. Fittingly, his zanpakuto's ability is Energy Absorption.
 * In Voltron, it turns out the the right knee where Black Lion and Blue Lion connect is the titular mecha's weak point. The enemy constantly bombards it when they find out about it before they're able to pull away and finish the fight before they lose the leg. However, in the end, they fix THAT weak point and put in another weak point! What the hell?!
 * Many villains in Yaiba has this. For example, Bat-Man's weakness to sunlight, the Moon Bunnies' ears and Silver's eye.
 * In Pokémon Special, Yellow's Virdian-blessed powers include healing, Telepathy, minor telekinesis, and Super Empowering. All this sounds like it would make her an invincible Pokemon trainer, but while her powers do make up for her sub-par battling skills, using them too much runs the risk of her being forced into a deep sleep that cannot be externally disrupted. At one point she gets taunted into using her powers vainly as to get tired out faster.

Comic Books

 * Perhaps the most well-known Achilles Heel is Superman's weakness to Kryptonite Factor. He's also perfectly susceptible to magical and (sometimes) telepathic attacks.
 * It's also generally accepted that Superman would lose all of his powers while exposed to red solar radiation, since the yellow sun is the source of his power.
 * Though writers sometimes waffle about whether red sunlight instantly depowers him or if he gets gradually weaker as he uses up his stores of yellow solar energy.
 * The unstoppable Juggernaut from X-Men is susceptible to a mental attack once his helmet has been removed.
 * Martian Manhunter (and indeed his species) was, then wasn't, deathly afraid of fire, which did, then didn't, and now does again melt him into a puddle.
 * The Ultimate Marvel version of Iron Man can't touch anything with his bare skin, even dust, without causing himself excruciating pain--hence the suit.
 * So how does he touch the inside of the suit?
 * Beg pardon. Hence all the drinking.
 * And Ultimate Professor X has one weakness... Stairs.
 * Traditionally, Green Lanterns have had trouble with the color yellow--except the first of them, whose power came from a different source, and was instead vulnerable to wood. Parodied by the parallel-universe "Green Guardsman" in an episode of the Justice League TV series, whose ring was powerless against aluminum.
 * One wonders what Rot Lop Fan is vulnerable to.
 * It would have been even funnier if he had been vulnerable only to British aluminium.
 * The alien symbiotes, from the Marvel Universe, that created such creatures as black-suit Spider-Man, Venom, Carnage, and Toxin, are vulnerable to "sonics"--essentially, extremely loud or high-frequency noises. Extreme heat is also a notably unpleasant weakness of theirs.
 * Not just extreme heat: there's one time when Venom has been defeated by a lit zippo.
 * Inverted with the supervillain Doomsday. If you kill him, he comes back, immune to what killed him last time (so, if you tossed him into space, he'd come back probably able to survive sudden changes in pressure; if you did it again, he wouldn't need to breathe the next time; and if you did it a third time, he might--eventually--freeze, or burn up, and come back immune to that).
 * The brilliant, amoral Thanos from the Marvel Universe has a particularly strange weakness: self-defeatism. As Adam Warlock points out in a Hannibal Lecture in The Infinity Gauntlet crossover, he subconsciously knows he doesn't deserve victory, and thus arranges his defeat without realizing it.
 * Best. Achilles. Heel. EVER.
 * Rising Stars features a character completely immune to physical damage. However, he still needs to breathe, and is killed by someone sneaking up behind him and throwing a plastic bag over his head.
 * More generally, in the third story arc,
 * The Gladiator, one of Marvel's many Superman expys, is exactly as powerful as he thinks he is, so he can be defeated by denting his confidence.
 * Norman Osborne, the current Big Bad of the Marvel Universe, has one rather serious Achilles Heel: he's a nutcase. As Osborne he's a twisted and brilliant Magnificent Bastard -- and a sociopath. As the Goblin he's so batshit crazy he makes his Osborn side look well adjusted. As a result, it doesn't take much effort to get a Villainous Breakdown out of him. At one point, just being reminded of Spider Man lead to Osborne pacing around in his office naked and ranting. And this is still saner than his Goblin persona.
 * General Zod's associate Ursa has all the regular Kryptonian powers and weaknesses, but her eyes can't stand bright light. Knocking off her protective goggles will briefly stun her as she clutches her eyes in pain.
 * The Eradicator (or at least when he reappeared after Superman's death) was like that, too - he couldn't take Superman's body and ended up forging a new one with the room around him. It wasn't perfect and he couldn't take in light properly, hence his shades. It's because of this that his battle with Steel turned in Steel's favor - he knocked off his shades fighting back.
 * Originally Spawn had his necroplasm suit which allows him to do pretty much anything, but with a countdown of energy which, when it hits 0, drags him to hell (thought once they stopped displaying the number the energy was at, became more of an Informed Flaw). He's also vulnerable to heavenly weapons in general.
 * Man-Thing was Nigh Invulnerable so long as he remained moist.

Fanfiction

 * In Aeon Entelechy Evangelion due to mobility reasons the joints in the armor (normal or subdermal) are less protected, and therefore exploited.
 * In Winter War,  shikai power is given the weakness that if he tries to use it against someone with a sufficiently great reiatsu- say, a high-ranking Espada- rather than killing them, he is overwhelmed with their power and effectively self-Brainwashed and Crazy.
 * John in With Strings Attached has to eat all the time; if he doesn't, he fades pretty fast. He's also vulnerable to shrill sounds and was completely put out of action when the Hunter's BFS started screaming.

Film
"Mahalik: I found their weakness. They're powerless without their heads!"
 * The mother of all Achilles Heels is the exhaust port on the Death Star in Star Wars, through which an invincible planet-destroying space station can explode into an enormous fireball because of a couple torpedoes from a small one-man fighter spacecraft.
 * Admittedly, knocking out its reactor core. Or, given that there was an explosion, instead of the station just going dead, knocking out the reaction controls, started a chain reaction (heh) which destroyed the station.
 * Possibly the silliest Achilles Heel in film history was in Signs, where the powerful aliens who had been wreaking havoc turned out to have a fatal weakness to ... water. Possibly, they shouldn't have invaded a planet two-thirds covered in water without protective suits. Just maybe.
 * Which is why some people think they were actually doing some kind of ritual hunt or adulthood rite.
 * An especially silly weakness given that H 2 O is the second most common molecule in the universe, after H 2.
 * Aliens being vulnerable to water qualifies as its own trope, given the sheer number of times it has been used in sci-fi, especially old short stories. Extra points if someone says, "How ironic, the unstoppable aliens were vulnerable to so common a substance!"
 * Independence Day. Not only is the alien mothership compatible with Mac OS, but despite millions of years of technological advancement, they don't have even the most rudimentary form of a firewall.
 * However, it is likely that in their culture, software hacking was unheard of.
 * In War of the Worlds, the aliens are entirely vulnerable to Earth bacteria and viruses. While new beings likely wouldn't be vulnerable to earth's viruses, bacteria would be able to wipe out any species without an adapted (or adequate) immune system. This is because the different ways that bacteria and viruses work. While it's stated to be the common cold in the movie, it could just be a "sinus infection" (bacterial). Any alien species moving to a different planet would be highly vulnerable to any and all new bacteria.
 * In League of Extraordinary Gentlemen Volume 2, the virus was specially tailored by Dr Moreau. Yes that Dr Moreau.
 * A blink-and-you'll-miss-it "Achilles Heel" of sorts appears in the Jim Carrey movie Liar Liar, where, faced with the prospect of "doing The Claw to [his ex-wife]", main character Fletcher Reed says, "You've found The Claw's only weakness--sub-zero temperatures!"
 * The Wizard of Oz: The Wicked Witch of the West's vulnerability to water.
 * Parodied in Scary Movie 3. The aliens are fighting the group (though just because strangling is how they say hello), when Mahalik decapitates one with a shovel and says:

"Officer Palumbo: Bullets! My only weakness! How did you know?"
 * Also parodied in Harold and Kumar Go To White Castle, during a psychedelic dream sequence.


 * Lampshaded and overdone ad nauseum in Mind Hunters. The plot of the movie rests on a FBI profiler killer killing other FBI profilers by presenting them in situations where they will have to fall for traps he's devised, because said trap is their weakness. . However, the killer's true Achilles Heel is
 * In Attack of the Killer Tomatoes, the eponymous tomatoes are defeated by playing the fictional artist Ronny Desmond's new hit song "Puberty Love," which causes them to shrink and become vulnerable to crushing.
 * Similarly, in Tim Burton's Mars Attacks!, the Martian attackers are defeated by playing Slim Whitman's "Indian Love Call," which causes their heads to explode.
 * Talos the Titan in Jason and the Argonauts has a near-literal version of this Trope.
 * That's the way it was in the original.
 * In the Disney film Hercules, Phil makes a reference to his old student Achilles and complains about "that stupid heel". He even goes to Achilles's statue, flicks the heel which cause the entire statue to crumble.
 * The Omnidroid in The Incredibles could only be harmed by its own weapons.

Literature

 * Leto Atreides II, the eponymous God Emperor of Dune, who combines his body with a sandworm to extend his life by thousands of years and gain immunity to almost every form of physical damage, also inherits the sandworms' vulnerability to water.
 * In the Chronicles of Chrestomanci by Diana Wynne Jones, the Chrestomanci are powerfully magic characters who are nearly invincible due to their powers, However, every Chrestomanci has one particular weakness.
 * In David Eddings' The Belgariad, it appears the Achilles Heel of the ancient and evil god Torak is to tell him nobody loves him. As everything he'd done through his existence was to force people to love him, this throws him into an Unstoppable Rage... which causes him to drop his guard. Cue impalement.
 * Although it can vary, werewolves are typically weak to silver, touching it causes severe burning.
 * In The Lord of the Rings, Sauron is incredibly tough, but if the One Ring he created is thrown into Mount Doom, he will lose all of his power, so that all he becomes is a little formless evil spirit incapable of affecting anything around him forever. Arguably, that's worse...
 * Tolkien goes to lengths to justifies this. The original point of the One Ring was for Sauron to gain control over the leaders of Men, Elves, and Dwarves, to whom he had (while in disguise) given the other rings of power. He had to put most of his strength into the One Ring, in order for it to be powerful enough to control the others, especially the three Elven rings.
 * Then there's Smaug, in The Hobbit. With his jewel-armoured scales he is well nigh invulnerable - except for one spot in his belly, not encrusted by bits of his treasure, and unknown even by Smaug himself until Bilbo discovers it.
 * An unarmored spot or weakly-armored belly is a common feature of many dragons in Western legend. (See "Fafnir" example, above.)
 * Smaug knew about the vulnerability of his belly, and had created what amounted to a waistcoat out of diamonds from his hoard. (How he created this, or attached them, is never explained.) It was this protection that was flawed.
 * Generally a Soul Jar is the weakness for anyone who uses it.
 * It's stated in the Harry Potter series that Voldemort's weakness is his inability to love.
 * And his reliance on and treatment of the horcruxes. They were supposed to prevent him from dying, and they accomplished that perfectly well. The problems were that: a) He was very lax with their protection at times (giving the diary to Lucius Malfoy, for example), because he had so many, and b) He couldn't tell when they were destroyed (although he initially assumed he would be able to, for some reason) and so left his guard down.
 * Voldemort's inability to love features prominently in all of the times he is beaten (as well as in some others):

Lily Potter's Heroic Sacrifice for the son she loves so much gives Harry a shield that is able to make a Killing Curse rebound.

Harry Potter and The Philosopher's Stone: That same shield actually burns Quirrell's skin when he has been possessed by Voldemort, rendering him unable to attack Harry due to pain.

Harry Potter and The Chamber of Secrets: His brotherly love for Ron and, to some extent, Ginny, pushes him to go down to the Chamber of Secrets to rescue the girl, and his respect for Dumbledore gives him the things he requires to kill the Basilisk, one of whose fangs he uses to destroy Tom Riddle's Diary.

Harry Potter and The Goblet of Fire: His parents' ghosts protect him when he has to run away from the cemetery after Voldemort's resurrection.

Harry Potter and The Order of The Phoenix: When he is possessed by Voldemort, just thinking of the recently deceased Sirius is enough to kick Voldemort out. Also, Dumbledore's love for him stops the Headmaster from killing Harry in the hopes of killing Voldemort (which wouldn't have worked due to the horcruxes.

Harry Potter and The Deathly Hallows: Most prominent in here. First, Snape turned spy for Dumbledore when Voldemort targeted Lily Potter, whom he had always loved. Then, Snape protects Harry and helps him to the best of his ability due to his love for his mother (his mistreatment of him comes from his being a look-a-like of James Potter) during all the books. Later, Harry sacrifices himself so that his friends and schoolmates don't die (and thus casts upon them the same shield his mother cast upon him). Much later, Narcissa lies to Voldemort about Harry being alive because she wants to see her son Draco (which is the signal that they are fed up with how Voldemort has treated her). And, in the end, it's Harry's ability to work with all of his friends that allows them to defeat the Death Eaters.
 * One of the 'Demon Headmaster' books featured a super logical computer which was defeated by all the children in the world yelling pointless anti-logic.
 * In Dan Simmons' Illium and Olympos the Trojan war is being deliberately recreated. Part of this involves Achilles being given a sort of quantum invulnerability that ensures he will only every be hurt by an arrow intentionally fired at his heel by Paris, which probably takes the cake for most specific and most inevitable Achilles Heel.
 * Fairy folk in the Artemis Fowl series are all susceptible to animal fat. Being exposed to animal fat will suck out all of their magic.
 * This becomes an important plot point in the final book in the Percy Jackson and The Olympians series. A character who bathes in the River Styx gains the "Curse of Achilles," literally making him or her a one-man army. However, since humans aren't meant to be invulnerable, the Styx will burn them away if they don't focus on one point on their body to be their mortal point--a literal Achilles Heel, just not at the heel. 's Achilles spot is the small of his back; 's is his left armpit.
 * The Steel Inquisitors in Mistborn are very dangerous customers, with Allomantic powers as well as superhuman strength and speed.
 * So what you're saying is:
 * Steel Inquisitors own this trope. In addition to the weakness above,  also kills them instantly.   uses this to  . Of course, when they show up again in the third book, they grew Dangerously Genre Savvy and armored up their weak spot.
 * Also,.
 * In Aaron Allston's Galatea in 2 D, Red comes from a painting of Achilles and has the heel.
 * In The Dresden Files Nicodemus is made invulnerable by wearing Iscariot's Noose, but
 * Fairies are consistently vulnerable to iron, even if they're powerful enough to be practically impossible to beat with anything else.
 * In the Sime Gen series Simes frequently kill Gens by sucking out their Life Energy, but any Gen can kill the Sime first if they know where the Sime's conveniently at hand weak spots are.

Live Action TV
""I mean, look at this guy, hm? Pretty much the only thing that could do damage to that thick, stony hide...is himself. Or, maybe, I don't know, a piece of himself.""
 * In Angel, where Angelus figures out the Beast's weakness rather cleverly:


 * Doctor Who does this a lot.
 * Cybermen were given an Achilles Heel in the story "Revenge of the Cybermen": gold dust would asphyxiate them. This was not a tremendously damning weakness, since gold is neither common nor easy to form into an effective weapon. In "Revenge of the Cybermen", the Cybermen were almost Nigh Indestructible even on a planet made entirely of gold, because the locals didn't have the technology to make an effective delivery system for the gold. As the series progressed, gold was promoted to Kryptonite status: in "Earthshock", the Doctor injures, but does not incapacitate, a Cyberman by grinding a gold badge into its chest, but by "Silver Nemesis", Cybermen can be killed by lobbing gold coins at them, or shooting gold-tipped arrows. Gold-tipped arrows are very effective against Cybermen, but, of course, totally ineffective against anything else. It is sometimes joked that had the original series gone on, it would eventually have been possible to destroy a Cyber-war-fleet just by saying "gold" at them. While not mentioned in the new series, a fake website connected to it hints that the Cybermen seen in "Rise of the Cybermen" and later episodes had the gold weakness worked out in the prototype stage. It was showed on a computer screen in that episode that the weakness had been worked out.
 * The Sontaran's 'probic vent'. "Back of the Neck!" Of course
 * The Slitheen, being composed largely of calcium, can be killed with acetic acid.
 * The Daleks were originally vulnerable to turning off the static electricity.
 * The Macra were helpless without their special gas (which made one wonder how they managed to force humans to start making it in the first place).
 * The Ice Warriors are vulnerable to (what else?) heat.
 * The deadly sea weed of The Fury from the Deep were vulnerable to Victoria screaming (luckily, she was good at it).
 * In addition to the gold allergy mentioned above, the Cybermen have been vulnerable to emotions in several appearances, and solvents in others.
 * Autons are also vulnerable to solvents and, later, anti-plastic... which sounds a lot like solvents, but now with new, improved, cooler name!
 * The "Seeds of Death" are vulnerable to water.
 * The evil computer in The Green Death was vulnerable to logical dilemmas.
 * The maggots in The Green Death weren't phased by bullets, bombs, or bug spray, but met a quick end after eating hybrid mushrooms meant to be a meat substitute.
 * Mandragora energy is vulnerable to being conducted away.
 * The Fendahleen are vulnerable to salt.
 * The giant octopus Kroll was vulnerable to... a small stick. Okay, a special stick.
 * The Mara is (are?) vulnerable to mirrors and happy thoughts.
 * The Sea Devils and Silurians are helpfully vulnerable to a deadly gas which is blatantly introduced at the start of "Warriors of the Deep", then conveniently forgotten about until the climax.
 * The Destroyer in "Battlefield" is vulnerable to silver.
 * Fenric is vulnerable to puzzles.
 * The Master leaves himself vulnerable to happy thoughts in "Last of the Time Lords". In addition, his biggest weakness throughout the series is... the Doctor, the one person he's never fully understood. (One episode of the classic series shows that his worst fear is a room full of the Doctor, laughing at him.)
 * In Six Million Dollar Man and The Bionic Woman, extreme cold could make the bionic heroes' parts stop working until they warmed back up.
 * In Heroes, Claire Bennet has a Wolverine-class healing factor that can repair her body to brand new after taking the unrealistically extreme amount of punishment that always comes her way. However, if an object is stabbed into a certain spot in the back of her head, she's rendered "dead" until it's removed, and the same goes for anyone who copies her powers. It's implied that if this part of her brain was removed or destroyed, she would die permanently.
 * The Blutbad from the series Grimm have a weak spot at the lower right part of their back. If hit there it will stun them for a few seconds.
 * Sheriff Lucas Buck's vulnerability in American Gothic is his third eye--if stabbed there, he could be killed. What makes this a particularly Egregious example is that not only is there no indication of this vulnerability until the next to the last episode of the show, but what could have been a very cool mythologically resonant plot point is instead wasted twice over: the person who reveals and exploits this weakness is a throwaway character we'd never met until the previous episode and the stabbing doesn't even take, since Buck is revealed alive in his grave at the end of the episode.
 * In Star Trek Deep Space Nine, Founders are almost absolutely immune to physical damage, vulnerable to BIG amounts of radiation or phaser weapons...
 * They also have to go into liquid form for 8 hours a day, much like humans need to sleep. In one episode, Odo was prevented from doing so, and started physically falling apart as a result.
 * Actually, only Odo suffered from that particular weakness, and only due to his inexperience and general lack of knowledge regarding shapeshifting. It was stated in the series that the Founders that came from the Great Link did not need to revert to their liquid form everyday.
 * Star Trek the Next Generation has Data, an incredibly strong, fast, durable, and intelligent android...with an off switch on his side. Flipping it renders him effectively dead until he's turned back on.
 * The Borg are near unstoppable forces of nature that assimilate and adapt to everything they come across, but Star Trek Voyager reasoned that if they can't assimilate it then they can't adapt to it, which is why they were so vulnerable to Species 8472. The Voyager crew, on the other hand, used scientific investigation and creativity, which allowed them to design a weapon to use against 8472.
 * They are also unable to adapt to blades: Worf killed them plenty with those and they never adapted.
 * Although not one related to the villain itself, the Goa'uld of Stargate SG-1 have force shields that can be bypassed by slower-moving objects. (Bullets won't work, but a thrown knife will). Later, the Replicators can only be destroyed with projectile weapons. Yes, their only weakness is getting shot. It makes sense in the show.
 * Specifically, energy weapons don't work on Replicators because the machines have energy shields resistance to practically all forms of harmful energy. However, projectile weapons like modern-era guns do their damage through kinetic energy, which causes the Replicators' relatively fragile bodies to shatter.
 * Each Monster of the Week in the short-lived Deadly Games generally could only be stopped in two ways and were otherwise totally invincible. The first varies from villain to villain but is usually elaborate or silly: shoot him with an arrow through the small target on their chest, drop a house on her (a mobile home worked in a pinch), etc. The second is always "foil their evil plan"; they don't get do-overs. Either method makes them vaporize instantly.
 * Mighty Morphin Power Rangers: The Warzord Cyclopsis was one of the best things Rita ever deployed, because its computer could adapt to each Zord and Zord formation used, (configuring the shields, targeting, weapons, armor-plate angling and such). However, quickly switching formations enough would overtax this system, causing the computer to lock up due to being asked to do more tasks at once than it was capable of. This allowed the Rangers to beat its restored form, obliterating it permanently.
 * Z-putties, anyone? In the first fight, the rangers stated they were invincible, until Jason discovered they explore when you hit the "Z" on their chest. Later, kids were able to destroy them launching balls at the "Z".
 * How about the Green Ranger after his return? Because he was the Sixth Ranger, he was plenty powerful, but because of how he returned to action, he couldn't stay in battle too long or his powers would run out. Come the second season, most of Zedd's early plots were essentially "take out Tommy."
 * Power Rangers Lost Galaxy had the Chameliac Warrior who copied the Rangers' powers... that is, until they swapped their attack moves with their friend's moves: Pink used Blue's moves, Green used Yellow's moves... you get the idea.

Mythology and Religion

 * The name for this trope is the Greek hero Achilles. In Homer, he's not invulnerable (he bleeds from a minor spear wound). Classical Greek art depicts Achilles being killed by an arrow to the body. But Statius, writing in the 1st century AD (900-some years after Homer), gave him a colorful superpower. According to Statius's account, Achilles' mother, the nymph Thetis, dunked him in the River Styx to make him immortal. However, she wasn't able to dunk the foot she held him with, since as a goddess, she could not bear the touch of the Waters of Styx. Quite naturally, he was killed by a poisoned arrow to the heel, courtesy of the cowardly Prince Paris of Troy.
 * Why didn't she just use a rope tied to his foot?
 * Better yet, she could have dunked him, pull him out, dry him up, flip him over and dunk the dry foot.
 * Also from Greek myth, the Gigantes were the children of Gaia and only had their supernatural powers when they were touching the ground. Heracles defeated at least one of them by picking him up with one hand and beating him with the other.
 * Antaeus is not labeled among the Gigantes. The true Gigantes did have a weakness though. They could only be killed by weapons of a mortal being. Hence, the gods invited Herakles (or perhaps that was the entire point about their affairs with mortal women). Some Gigantes were defeated by gods, but not killed - one is still alive, breathing fire under Etna.
 * Mythology loves this one. In the Persian epic The Shahnameh, the Persian prince Esfandyar bathed in a pool of invincibility, but kept his eyes closed as he submerged himself. When he picks a fight with the hero Rostam, he gets killed by an arrow through his eyeballs.
 * In the Biblical Book of Judges, Samson loses his Super Strength when Delilah cuts off his long hair. Notice that this is hinted to not be a 100% Achilles Heel. Hot Blooded as he was, Samson had broken all of the other rules God gave him to keep his Super Strength and telling Delilah about his hair (and how he was consecrated to God) was figuratively the last straw.
 * In a nutshell, the big three vows require Nazarites to abstain from alcohol, to not cut their hair, and to avoid corpses. Apparently Samson was known to have had wine in the past, touched (and consumed honey out of) a lion's corpse, and well, there was that whole 'beat hundreds of Philistines to death with the jawbone of an ass' bit, which doubtless left him around a number of dead bodies afterwards.
 * The Philistine thing was actually a ruse to have a pretext to attack them, ie, a sting operation.
 * When Baldr of Norse Mythology started dreaming of his own death, his mother went around to every single thing in existence and made them swear an oath to never harm Baldr. Presto, invulnerable god, right? Not quite: she forgot to ask mistletoe, since it seemed too small and shriveled to be a threat. When the rest of the pantheon decided to make "Let's throw things at Baldr" into their favorite party game, Loki (who was jealous of the attention accorded Baldr) tricked the blind Höðr into shooting a Mistletoe arrow at Baldr...
 * In the German poem, the Nibelungenlied, the hero Siegfried bathes in the blood of a dragon after he's slain it, and becomes invincible as a result. However, a leaf had stuck to his shoulder, and thus left a spot there untouched by the blood... and yes, the similarities between him and Achilles have been noted before.
 * The Tlingit people of the Pacific Northwest have a monster with an Achilles Heel: Once there was a giant that ate people. A man decided to kill the giant, so he played dead where the giant walked every day. The giant found him, brought him home and left to get firewood. The man jumped up, threatened the giant's son with a skinning knife to make him tell where his father's heart was. It was in.... The left heel! The giant returns, and the man stabs his heel. The giant screams, "I will eat your people forever!" But the man chops him into pieces and burns him. The ashes become mosquitoes, which form into a cloud and laugh, saying "I told you: I will eat your people forever!" Then the man looks on his arm, which has a mosquito on it...

Sports

 * Basketball player Shaquille O'Neil (or Shaq), one of the best players in recent NBA history, had an infamously poor free throw percentage. The best way to keep Shaq from steamrolling you was to intentionally foul him, forcing him to shoot free throws that he'll probably miss. This tactic became known as "Hack-a-Shaq." Interestingly, Shaq was a consistently decent-to-good free throw shooter in practice throughout his career; something about the pressure of being put on the spot in a game situation always threw him for a loop.

Tabletop Games

 * Many Dungeons and Dragons monsters have specific weaknesses that either do more damage than normal, or are necessary to even be able to kill them. The most famous is arguably the troll, which in most editions can't be killed with just standard weapons, but that also needs to be burned with acid or fire for it to permanently die. Unconventional tactics can also exploit certain weaknesses, or at least find other ways to kill them.
 * Also included are demons/devils vulnerability to certain metals such as cold iron and silver
 * Virtually any named enemy in Deadlands will be Nigh Invulnerable, except for one weakness somehow obscurely related to . The list goes on: Stone, Raven, the Bishop, prominent Whateleys,  ... In fact, it's so common that one of the most coveted abilities of arcane characters is the rare ability to intuit these weaknesses. (That's part of why the Badass Normal posse keeps the bookworms around.)
 * Champions was the first ever Tabletop RPG to feature intentionally taking weaknesses (called "Disadvantages" in the rules) as a part of the character creation process. The disadvantages "Susceptibility" (which causes a character to take damage from something other characters find harmless, such as Superman's reaction to Kryptonite or a vampire's taking damage from holy water) and "Vulnerability" (which multiplies incoming damage from certain types of attacks, such as a werewolf's vulnerability to silver) were specifically created to mimic an Achilles Heel.
 * GURPS copied the Vulnerability disadvantage from ''Champions.
 * Most Tabletop RPGs based on Comic Books (Villains and Vigilantes, Mutants and Masterminds etc.) have followed Champions lead when it comes to weaknesses as a standard part of their character creation rules.
 * Shadowrun has the Allergy disadvantage, which comes in three grades, depending on the effect of contact with the substance: mildly annoying, exceptionally discomforting, and physically harmful. Shapeshifters automatically have Severe Allergies, typically to silver, vampires have Severe Allergies to sunlight and Moderate Allergies to garlic, etc. The really silly thing is that the rulebook has some recommendations for allergies, which include seawater and oil.
 * Yozis in Exalted can be sent through an agonisingly painful and debilitating death-and-rebirth cycle if you take out their fetich soul. It is worth noting, however, that this is often much harder than it sounds.

Video Games

 * Just about any glowing spot, on any Boss Battle, in any video game. Sometimes just does extra damage if hit, but is often (in Platformers especially) the only place where the boss is not Nigh Invulnerable.
 * In many First Person Shooter games, enemies may have strong body armor, but can be killed faster by shooting them in the head (i.e. headshots).
 * For example: House of the Dead, where not only do headshots take down enemies faster, but each boss has a specific weakpoint, and cannot be damaged (or only take minimal damage) if shot anywhere else. The best one is the one with a huge gaping hole in its chest that opens to reveal its heart every time it beats (How...not easy...).
 * The superhero MMO City of Heroes balances all classes and powersets by giving them strengths and weakness, but only the Peacebringers and Warshades have a specific crippling weakness, Quantum Energy damage, which only NPCs with Quantum Weapons can deal. The problem was that the developers overdid it and made the weapons available to every enemy group in the game, essentially distributing kryptonite bullets, and it many cases it was powerful enough to kill the player in a single hit. As this was a major gripe about the two classes, eventually the developers responded by reducing the effects of the weapons to make it simply a danger rather than an overwhelmingly crippling threat.
 * Furthermore, many defensive sets have Achilles Heels in the form of damage types they offer only minimum protection against. Ice Armor melts under fire damage, Fiery Aura is susceptible to cold, Dark Armor has only minor protection against energy and so on. Only two or three armors in the game offer appreciable protection against psychic attacks.
 * Ironically the Quantum Energy problem almost exactly mirrors a problem encountered by Superman.
 * The Legend of Zelda. In this case, it's nearly always the eye or another insufferably obvious 'weak point'.
 * Usually an eye in an odd part of their body.
 * Ganon/Ganondorf usually has a spot on his chest or his weakness is his tail. And of course, the Master Sword also counts.
 * In the later editions of the Resident Evil series, bosses began to show certain specific weak points. For example, all of the Uroboros creatures in RE5 have glowing weak points which can be shot to expose bulbous body parts that can then be targeted to kill the creature itself.
 * Resident Evil 4 gives two main ways of dealing with Krauser. Shoot him in the knee so he drops his shield, then shoot him in the head, over and over again. With indifferent aim it's easy to run out of ammo before Krauser is killed by gunshots. Or stab with the knife, normally a weak Emergency Weapon, about 10 times.
 * "Please do not prod my Achilles buttocks." The above line is delivered by the Maître d'Bot (in a hilarious French accent) when you poke him in the butt with the stick. You have to do this because it's the only way to get to the corpse holding a fuse you need to repair the ship.
 * In the Pokémon games, Shedinja is immune to 12 of the 17 attacking types. However, it will always get knocked out quickly when hit by an attack of those 5 that can hit it because it has just one hit point at any level.
 * Several forms of indirect damage will also defeat it, like switching in when the widely used Stealth Rock is up.
 * Oddly, this leads to Shedinja being used in both the lowest and highest tiers in competitive battling. Those 5 types and Stealth Rock (especially in overused) are much more common in the middle and low tiers. However, in the lowest tier it actually has stats that don't completely suck compared to other Pokemon.
 * In Lost Souls MUD, great power notoriously often comes with great vulnerability to holy damage.
 * Every RPG ever with a Barrier Change Boss. Figuring out what the new Achilles Heel is in order to even make a dent in the boss (or avoid healing it) is the entire point of the battle.
 * The Trope Namer appears in Warriors Orochi, but this trope is averted. In no way you can attack Achilles on the heel, and you can beat him by just beating the crap out of him on any body parts.
 * The Reactor in Might and Magic VI has a very simple Achilles Heel: it only takes damage from blasters. This is, in fact, blasters greatest advantage: their damage ignores any resistances or immunities possessed by enemies, and as the final dungeon not only features the Reactor you need to destroy, but also hordes of enemies with high resistances to every other sort of damage...

Visual Novels

 * Arcueid Brunestud, and presumably all the True Ancestors from Tsukihime. Normally Arc in particular is all powerful, but thanks to the protagonist Shiki cutting her to pieces early on in the game she's usually pretty weak. We finally see her true abilities at the end of the Ciel route, when
 * Gilgamesh in Fate Stay Night has the two most broken Noble Phantasms in the game. One of them is beamspamming Noble Phantasms and the other is some spinny drill sword thing Ea, an anti-world Noble Phantasm that absolutely dominates Excalibur. Oh and he can use them both all he likes. Weakness? He's an Archer, not a swordsman, so all he can do with those swords is beamspamming them instead of using them properly. Much more importantly, he has an ego so larger that it's even bigger than his ego. Wait. But anyway, he never takes any fight that he's in seriously until he's in mortal peril. Or pieces.

Webcomics
"Black Mage: I found out what zombies are weak against.
 * In Sluggy Freelance, Aylee's crab-like transformation is nearly invincible. Her shell can take just about anything the characters can throw at her, but she can be hurt along her extendable neck.
 * Horribus is also only vulnerable on certain parts of his body. Even when Torg uses a magic, kill-anything sword, he can still only take Horribus down by either decapitating him or stabbing him right through his soul (located roughly in the center of his chest).
 * Frans Rayner in The Adventures of Dr. McNinja is vulnerable on only one place on his body (his right butt cheek). However, the protagonist (who witnessed his defeat as a boy) strikes the same place 25 years later and fails... Rayner had moved it to his left eyebrow. Then in yet another encounter, it's revealed that following his previous defeat, Rayner moved it again, into his right leg, which
 * From Eight Bit Theater, "A spear? Through my brain? My...only...weakness..!
 * Parodied similarly much earlier on when Black Mage is left to deal with a zombie orc. After an explosion propels him back in front of the team:

R Mage: Oh?

Black Mage: Point blank annihilation."


 * Similar to the Z-Putty example, Chris-Chan Sonichu of Sonichu had his main weakness be the white markings on his chest. However, it was rarely struck in the entirety of the series, making it a moot point.

Web Original

 * In Darwins Soldiers, Dragonstorm experiments' only weak point is the back of the net, since this is where their neural net connects to the rest of the body. Said weak point is ridiculously hard to hit, however, since they're usually running towards you, and they're naturally armored.

Western Animation
"Robot Army Officer: "How ironic; the Human was immune to our most powerful magnetic fields, yet, was vulnerable to a harmless sharpened stick.""
 * You'd think, with the Omnitrix making him able to transform into an alien of his choice at will, that Ben Tennyson of Ben 10 would be pretty much invincible. He isn't though, for several reasons, including but by no means limited to: the seemingly random time limit (possibly based on some measure of energy expenditure, as the Omnitrix generally needs to 'recharge' for an equally random amount of time afterward), the Omnitrix's glitches turning him into an alien other than the one he chose, his own lack of knowledge of the forms hidden in the Omnitrix and the full extent of each one's powers, and, perhaps most dangerously, having the mindset of a ten-year-old boy.
 * When Alex O'Hirn becomes the superstrong Rhino in The Spectacular Spider Man, he goes on a destructive rampage and appears completely unstoppable. His Achilles Heel turns out to be the same thing that makes him so tough: Because his suit is completely impenetrable, he can only perspire through his exposed face. Spider-Man is able to defeat him by trapping him in a steam-tunnel in the sewers, which causes the giant thug to overheat and collapse.
 * In Justice League, the forge god Hephaestus says that all of his creations are given one weakness so no one can become all-powerful. When Wonder Woman asked him what Ares' Humongous Mecha is vulnerable to, he doesn't tell her, saying she wouldn't want him going around telling the weakness he'd placed in her items. She does not look happy the revelation that she has any.
 * I think Wonder Woman was a bit offended by the implied sexual innuendo of "the weakness of your armor". Hephaestus definitely makes the line sound a little lecherous.
 * The weakness is that if you bind her wristbands together she loses her super strength and is rendered helpless.
 * The weakness of the aforementioned Humongous Mecha that Hephaestus made for Ares was...  In a later episode, this flaw was removed by.
 * In Avatar: The Last Airbender, Firebenders have no powers during the solar eclipse.  Likewise, Waterbenders are powerless without a source of w close by, although sweat has been used, w was drawn from the moisture in the air, and with the full moon behind them they can also bend blood (there are moral implications to that though). It would be expected that tears and saliva could be used too. Earthbenders are powerless if separated from earth, sand or stone, unless you can  . The only Avatar race that's mostly invulnerable are the Airbenders since air is always around. And yet they were the ones that ended up getting wiped out. Go figure.
 * Possibly justified: the Airbenders were almost all peaceful nomads, so the Fire Nation took them out first, possibly because they were the most versatile in their bending.
 * Or it could have something to do with the fact that they knew the next Avatar in the cycle would be born an airbender...
 * Or maybe both.
 * And the fact that the firebenders were being powered up by Sozen's comet, which gave each one of them the strength of a hundred.
 * It was explained in the first season that airbenders have unlimited source to their element, but since it is air they don't have much power behind it, unlike waterbending or earthbending. Aang was once shackled with chains and no amount of airbending could break them.
 * However a true Master Airbender could easily ruin a Firebender's day by creating a vortex such as a tornado removing burnable amounts of Oxygen. Peaceful never means weak.
 * Also, Aang, when he enters the Avatar State, unleashes the power of all the Avatars before him and unlimited cosmic energy. However, if killed in the Avatar State, the Avatar Cycle stops.
 * The concept was portrayed hilariously in the movie Fry and Leela watch in the Futurama episode "Fear of a Bot Planet". Taking place on a planet where humans are considered monsters, and the citizens are all robots:


 * Also Lampshaded in Bender's Big Score, during the battle against the Scammers' fleet of gold-plated Death Stars. One of them is destroyed in classic "fly-in-and-blow-it-up-from-the-inside" style, with access being gained via a hatch labelled the Achilles Port. By Al Gore of all people.
 * "Finally! I get to save the world with deadly lasers instead of deadly slide shows!"
 * The members of the Sushi Pack lose their powers and strength if they're exposed to heat. They also tend to announce this whenever it happens. "Heat! Our only weakness!"
 * In a Harry Potter-themed Treehouse of Horror Montymort's (Mr. Burns) only weakness was his enchanted shin.
 * Parodied in another episode of The Simpsons, in which Homer is bribed with a cake and he laments Marge knows his one weakness, that he's weak.
 * From Gargoyles, the Archmage during the "Avalon" three-parter has gone from a formidable but very overconfident mortal sorcerer to a demigod powerful enough to command the allegiance of three of the most powerful Children of Oberon in the series. The catch? He can only control his incredible new powers by using the Eye of Odin, and when Goliath manages to get it away from him... well, let's just say he sufferes Power Incontinence so bad it kills him.
 * Something of a Justified Trope with the introduction of the combiner super-robot Bruticus on The Transformers in the course of the episode "Starscream's Brigade." While he's first shown thoroughly smacking around fellow massive combiner Devastator, he eventually turns on Starscream and Megatron to very littles surprise. However, Starscream, with unusual foresight in regards to treachery, had left three weak spots on Bruticus' back, where a good shot or two would force the big guy to fall apart into the individual robots that comprised him.
 * Used and then some during the run of the Japanese Headmasters cartoon in the G1 era. Scorponok, one of the Decepticons' largest and most powerful warriors and leader figures in the course of the show, was positively massive even to the already giant-by-human-standards Transformers themselves. Scorponok's weakness was revealed later in the run of the show by the engineer who'd designed his body. Its vulnerability? A weak spot... directly behind the enormous Decepticon insignia he wore. Dead center on his chest.

Real Life

 * Being one of the most famous memes ever, Achilles' heel had a body part named for it, namely, the Achilles Tendon. Sever it, and you won't be walking again for a long, long time.
 * Tendons in general are strong but have very little blood supply and heal very slowly. They also get stronger in response to exercise very slowly compared to the muscles they connect to. As such, a common and serious source injury of people who have taken up a new sport and don't know their limits to have their muscle strength in the required muscles become sufficient to injure the weaker tendons they connect to.
 * Not to mention joints, eyes, pressure points, the throat, the groin and the spine, especially near the base of the head are all weak points that martial artists take advantage of to one shot their opponents, it doesn't matter how strong you are if someone hits you in the joy zone.
 * While most Nintendo products are literally capable of taking a head-on tactical airstrike and still functioning perfectly, most of the DS line features an Achilles' Heel in the form of the hinges, which are not only destructible, but quite vulnerable and can in fact eventually break from overuse.
 * The cobra is a Badass deadly snake from India with only one weakness. It strikes opponents rather slowly. Thus, the agile mongoose is able to defeat the cobra by jumping out of the way when it strikes and leaping up to bite the head when the snake is drawing back from the strike. Believe it or not, the cobra often manages to bite the mongoose once, but since the mongoose has a partial immunity to the snake's venom it takes more than one bite to kill one, and the cobra usually tires out after it manages to bite once. In any given confrontation between an Indian Cobra and an Indian Grey Mongoose, the mongoose tends to prevail three out of five times.
 * Mongooses in turn have an Achilles Heel when it comes to the kinds of snakes they can kill. Cobras are fast-moving snakes, but they have slow and predictable strikes, which is their Achilles Heel. In contrast, vipers are sluggish, slow-moving snakes with wickedly fast and unpredictable strikes. That's why, in a fight against a viper, the mongoose loses 95% of the time.
 * Additionally, mongooses have another Achilles Heel in that they are only resistant to the venom of cobras and their close relatives. This leaves them susceptible to the venom of other poisonous snakes that they share their range with, such as the black mamba.
 * There was once an attempt made to reduce populations of fer de lance vipers on an island by introducing a population of mongooses. It was a disaster. The mongooses were all decimated by the snakes and domestic cats were used instead.
 * Crocodiles and alligators have an Achilles Heel in their jaws. While the muscles they use to bite are very strong, the opposing muscles used to open their jaws are much, much weaker. A decent elastic band can turn a crocodile from deadly prehistoric monster to walking handbag. The same is true on a less impressive scale with crabs and lobsters.
 * Sharks have a similar weakness: if you manage to flip one upside down, it will enter a state of "tonic immobility" and become paralyzed. Some orcas managed to figure this out to hunt sharks.