Takes One to Kill One

""Magic must defeat magic!""

- Uncle, Jackie Chan Adventures, explaining for the umpteenth time why you just can't punch out Cthulhu or shoot him.

In many works of fiction, there are oftentimes creatures that are Immune to Bullets, Made of Iron, and in worst-case scenarios have no Kryptonite Factor. Even nuking them has no effect. But fear not, for while these creatures may be immune to fire, bullets, nuclear weapons, and MTV, these monsters are still have one weakness...another member of their own kind.

This is oftentimes used as a justification to why the protagonists, who oftentimes either are this kind of monster, can turn into one, or have one under their control, can seriously wail on the Monster of the Week, and yet the military forces and police are still useless. If the monster is unaffiliated with either side, it can lead to Always a Bigger Fish. If the protagonists are forced to become this type of monster to win, expect a My God What Have I Done moment afterwards.

A subtrope of Mutual Disadvantage. An opposite of Tactical Rock Paper Scissors where each type of combatant has some other capable of curb stomping it. Compare Beat Them At Their Own Game, Hoist By His Own Petard and Summon Bigger Fish. Contrast Like Cannot Cut Like.

Anime and Manga

 * In Neon Genesis Evangelion, the Evas are the only things on Earth which can fight an Angel and win, because only they can break the Angel's AT fields.
 * To be more accurate, the Evangelions are the most easily deployed and most reliable way to defeat an Angel. AT-Fields can be pierced by raw force, however. Ramiel, for instance, was defeated by charging up a positron cannon with enough juice to just blow a hole in him AND his AT-Field, all EVA-01 did was serve as a weapons platform as the cannon was unfinished and therefor had no deployment apparatus. Certain other options exist to hit the 180 million megawatt figure stated to be necessary to overcome Ramiel's AT-Field, which was one of the strongest of all the Angels (behind Tabris and Zeruel, most likely). The largest nuclear devices, for instance, could likely knock out an Angel, though nuking them isn't exactly an ideal solution.
 * In The Big O one of the few things that can damage a Megadeus is another Megadeus.
 * Ryo Takatsuki's nanite-powered Super Powered Evil Side in Project ARMS has a unique power to negate the Healing Factor of other ARMS.
 *  ~Blood: The Last Vampire~  has this as its basic premise.
 * Same goes for its spin-off series Blood Plus. Chiroptera are only vulnerable to the blood of an opposing queen, and about the only thing that can kill a Chiropteran queen is another queen.
 * The plot of Eternal Sabbath revolves around getting Shura to kill Isaac, as Shura is the only one who understands Isaac enough to stop him.
 * In Yu-Gi-Oh! ZEXAL, the Numbers monsters cannot be destroyed in battle except against another Number. Subverted in that they can still be destroyed by card effects, and it's entirely possible to negate their invulnerability so that they can be destroyed by any monster.
 * In Baccano!, the only way an immortal can die is to be absorbed by another immortal.
 * Puella Magi Madoka Magica seems to play with this trope. At first, the Puella Magi magic appears to be the only way to kill Witches (straight trope), but it is eventually revealed that  (subversion)  . And then, in episode 11,   (thus, double subversion).
 * In Heroic Age, all Nodos are so exceedingly powerful that the only thing that can fight a Nodos is another Nodos. Anything else is certain to get curb stomped. Even then, battles between Nodos can go on for quite a while. At one point, a Nodos battle lasts for 300 hours straight.
 * In Heroic Age, all Nodos are so exceedingly powerful that the only thing that can fight a Nodos is another Nodos. Anything else is certain to get curb stomped. Even then, battles between Nodos can go on for quite a while. At one point, a Nodos battle lasts for 300 hours straight.

Comics

 * Superman's immunity to bullets and most other forms of bodily harm come from an innate forcefield he projects around himself. However, other Kryptonians are able to pierce this forcefield and harm him.
 * He's vulnerable to Kryptonite, a material from his home planet, meaning he is, in a way, vulnerable to his natural environment.
 * Radioactive material from his natural environment no less.
 * When the New 52 version of Supergirl first fights the Worldkillers, one of them boasts that only a Worldkiller can defeat a Worldkiller. Supergirl exploits this by grabbing one Worldkiller's Combat Tentacles and stabbing another one with them.

Film
""Only a ninja...can destroy a ninja.""
 * In The Incredibles, Mr. Incredible finally realizes that the only thing that can defeat the Killer Robot
 * Ninja 3: The Domination


 * In virtually all kaiju films, especially those with Godzilla and Gamera, the only thing powerful enough to take down a giant monster is...well, another giant monster.
 * In ~Mega Shark vs. Giant Octopus~, the entire U.S. Navy couldn't take down one of these creatures. The plan? Have them kill each other!

Literature

 * Nicodemus, one of the Rogues Gallery of the Dresden Files, is protected by the noose that hung Judas, which regenerates damage from any cause... except for itself.

Live Action TV
"So you need one to kill one, but you got to kill one to make one. How does that work out?"
 * In Angel season 4, Angelus figures out that the only thing that can harm the Beast is a knife crafted from its own rocky hide. Which it had conveniently crafted already as an offering to its master.
 * One of the spells in Charmed for defeating a ghost only works if it's said by another ghost. Fortunately, becoming a ghost doesn't require that you actually die, just be unconscious and near death.
 * On Supernatural according to lore the only way to kill a dragon is with a sword forged with dragon's blood.


 * The only way to permanently kill Leviathans is for them to be eaten by other Leviathans. They can be killed via decapitation, but they only stay dead as long as the severed head is kept separate from the rest of the body. If the head is left close enough to the body, it will reattach and the Leviathan will come back to life.
 * One episode of Xena: Warrior Princess dealt with a villain making weapons out of Hephaestus's metal, which can only be damaged by other weapons made of Hephaestus's metal.

Mythology

 * Older Than Feudalism example: In a poem ascribed to Theocritus (3rd century BCE), Hercules discovered that the only thing that could pierce the Nemean Lion's hide was its own claws. Hercules had already strangled it to death by this point -- he just wanted to skin it.
 * In an undated Greek Mythology, the twin giants Otus and Ephialtes were so strong they could only be harmed by one another. Artemis tricked them into shooting one another with their spears, killing both brothers simultaneously.

Other

 * A lot of werewolf and vampire fiction states that one of the few things werewolves and vampires are vulnerable to (except, you know, silver, stakes, and such) are bites and scratches inflicted by another werewolf or vampire.
 * Alucard and Blade are both vampires who hunt vampires.
 * Van Helsing: The only thing that can kill Dracula is a werewolf.
 * In the Old World of Darknessuniverse, werewolf claws do aggravated damage, meaning that they are one of the few ways to easily take down another werewolf or any variety of supernatural beasties.

Tabletop Games

 * In Dungeons and Dragons 3rd edition and 3.5, creatures with damage resistance have their natural weapons (if existing) able to pierce that same resistance, so every creature is capable of killing its own species.
 * In 4th Edition Dungeons and Dragons, The Prince Of Hell Epic Destiny gets immunity to fire. He also gains the ability for his fire attacks to ignore resistances / immunity to fire, meaning only he can burn himself.
 * In Werewolf: The Apocalypse, one of the setting ending scenarios has the Wyrm having become so powerful that only its own weapons can even hurt it.

Video Games

 * In Breath of Fire: Dragon Quarter, dragons can only be harmed by other dragons. This means that if Ryu transforms in a conventional battle (including the vast majority of boss battles), he is invincible. There is one exception: A certain enemy dragon can be harmed with conventional weapons if a transformed Ryu attacks first and "breaks the defense".
 * Iji was enhanced with nanomachines in order to combat the threat of alien invaders, because nanoweapons are about the only way to even scratch the alien's armor, much less go up against one and win.
 * And the final boss must be taken down with his own projectiles.
 * In Persona 3 and Persona 4, Personas are actually . As such, they can (and have).
 * Perhaps rather famously, the Dragon type in Pokémon, though it is also weak to Ice. Generally, this a better bet anyway, since fighting dragons with your own dragons makes them into Glass Cannons, naturally.
 * Same with the Ghost type, though they're vulnerable to a number of other types as well.
 * "???"-type enemies in Alter AILA Genesis are the enemies who use Psych-elemental attacks the most often, but they are also the only enemy weak to it.
 * Many other enemies do this too; robots use Shock attacks the most, and humanoids tend to use Force, both of which are the elements they're most weak to.
 * Metal Man from Mega Man 2 can be one hit killed by its own weapon.
 * In Final Fantasy Tactics, the "Faith" stat dictates not only a mage's attack power, but how much damage they incur from enemy spells as well. A character with very low Faith is thus virtually immune to magic, while a character with high Faith is very vulnerable.
 * In Paper Mario the Thousand Year Door, during the Inevitable Tournament, one of your enemies is a pair of metallic, spike-covered critters... you can't hurt them with jumping-attacks, hammers, fireballs, or anything else. In fact, it's a Hopeless Boss Fight... until Yoshi Jr. joins you for the rematch. By eating one and spitting it at the other, he's capable of damaging them - because, as it turns out, their only vulnerability is themselves.
 * Dragons of Skyrim are otherworldy beings bound to flesh. Upon death, their souls do leave their bodies, but can easily return with help from another, making them functionally immortal. The only way to truly kill a dragon is for another dragon to devour their soul. The player character, a mortal born with the soul of a dragon, is close enough to a real dragon to pull this trick off themselves.

Web Comics

 * In Homestuck, it's believed that one way to kill a nigh-invulnerable First Guardian (or a creature possessing its powers) is to send another to fight it.

Western Animation

 * Courage the Cowardly Dog had the Kangaroo Monster, a creature so Badass the only thing that could harm it...was another Kangaroo Monster!
 * Jackie Chan Adventures: "Magic must defeat magic!"
 * Essentially Magic Versus Science where Magic a Is Magic A and therefore unaffected by Science B.
 * On South Park the only way to defeat a giant stone Abraham Lincoln is with a giant stone John Wilkes Booth.
 * Also, immortal beings like Cthulhu can only be killed by other immortals.
 * In a similar situation in Dexters Laboratory, Dexter uses a giant stone Abraham Lincoln (who shoots missles out of his hat!) to fight Mandark's giant stone George Washington. However, the trope is subverted when the two stone presidents find out they are honest men who have a lot in common, stop fighting, and walk off as friends.
 * In another episode Dexter gives Dee Dee what he thinks is a silencing potion, but instead mutates her into a monster. The only way to get her to shut up? To become a giant monster himself and have a knock-down, drag-out kaiju battle!
 * Parodied in a "What If" episode of Futurama, where the Planet Express crew creates a Giant Zoidberg as only an "even equally big" monster could defeat a 500 foot tall monster rampaging through the city. This fails miserably as Giant Zoidberg immediately goes on a power trip and rampages through the city, and so the two giants fight and kill each other in the ensuing turf war.
 * In the second movie, The Beast With A Billion Backs, electromatter can only be damaged by other electromatter.
 * Ultimate Avengers provides a technological example: vibranium armor can only be penetrated by vibranium weapons.
 * Nuclear warheads and The Hulk also work, but those tend to be more trouble than they're worth.

Real Life

 * British forces in the Battle of the Atlantic were commanded by a submariner.
 * Frequently subverted by the actual real life military. Often, the best way to kill or disable a hostile unit is using something completely different against it; for example, battleships fell out of favor after WW 2 because too many of them had been destroyed by aircraft, and a tank's worst enemy isn't necessarily always another tank.
 * That's often because the best use of tanks is in destroying supply lines, rather then because tanks are not effective against other tanks.
 * Diamonds can only be scratched by other diamonds.