That One Boss/Role-Playing Game/Pokémon

Behold the tales of Pokémon bosses, Gym Leaders, and Grand Masters for whom lolz at the player's expense flow more readily than the water in the Eclo Wastes.

NOTE: Final Boss and Wake Up Call Boss can't be added unless they are overpowerful by their standards. Bonus Boss is now completely banned, as they are optional and their standards of power can't be measured.

Generation I: Red, Blue and Yellow, plus FireRed and LeafGreen

 * Misty's Starmie would qualify since it's effectively an OU strong Pokémon fighting against unevolved Pokémon. Starmie is guaranteed to be faster and stronger than anything you will have at that point and it does very high damage even for mons that resist Water. It lacks a Psychic attack, but doesn't even need it.
 * It Got Worse in FireRed and LeafGreen, where her Starmie has switched BubbleBeam for Water Pulse which confuses you about once every three turns it's used. As we all know, if the opponents use a move with an effect, it will almost always instantly activate. If you became confused, you were more than likely to hit yourself and faint in the process, which means that if you get confused, you should reset the game and avoid a lot of trouble by starting the match over again.
 * Lt. Surge was fairly easy in Red and Blue, but in Yellow he became a monster. His Raichu is level 28, which towers over anything else you've faced. Speed? It goes first. Offense? STAB Thunderbolt will massacre anything in its path, and Raichu also knows Mega Punch and Mega Kick to take care of anything else, even the frail Diglett if you thought to be smart and use one.. He has no other Pokémon, but why would he need any? Fortunately, he's not as much of a pain later on, in the Gen IV remakes you can just spam Earthquake against him.
 * Erika might not seem like much trouble. However, if she manages to get the first attack in, be prepared for her to spam the single most annoying attack of Gen 1, Wrap. Unlike the version of the attack you're now familiar with, Gen 1 Wrap prevented your Pokémon from taking any actions while they were caught. And guess what she'll do the moment your Pokémon is free from her Wrap attack. That's right, use Wrap again, preventing you from doing anything more than watching as the attack chips away your helpless Pokémon's health at a painfully slow pace to add insult to injury and God forbid she manages to paralyze your Pokémon while they're suffering wrap. You'll essentially be stuck in an infinite juggle.
 * Sabrina at Gen 1. Her team of Psychic-type Pokémon is fifteen to eighteen (depending on which version you're playing) levels higher than the last gym leader. It doesn't help much that Psychic-types were also extremely overpowered in Gen I due to programming glitches and a poorly-balanced elemental system in general.

Generation II: Gold, Silver and Crystal, plus HeartGold and SoulSilver

 * Whitney from Gen II -- her Miltank, to be precise. It has an attack called "Rollout" which gets stronger on subsequent use, and if it gets rolling, you're dead. And assume death is imminent should you have picked Cyndaquil for your starter, who's weak to Rock-type attacks. It has Attract, which will stun your male pokemon 50% of the time (bear in mind that male Pokemon are usually more likely to appear than female, depending on the species). It has Stomp, which is a strong attack (at least at the time you'll be facing her) that can flinch your Pokemon. So you finally got it down to red health? Miltank's got Milk Drink, which restores nearly all of its HP. This song sums it up nicely.
 * Upped to truly ridiculous levels in HeartGold & SoulSilver. Catching a Gastly to nullify Miltank's physical attacks doesn't work anymore -- they gave her the ability "Scrappy" (how appropriate) which allows it to hit Ghost-types with Normal-type attacks. On top of that, they gave it a Lum Berry as a hold item, so inflicting a status ailment on it will just make it heal itself. Sure, you can inflict the status a second time and it'll stick...assuming the status-inflicting Pokémon survives a second turn. At least Whitney's two Pokemon have dropped one level in the remakes. It doesn't help that much, but it does mean slightly less in each stat.
 * This is even referenced in the anime, where Ash takes Whitney on in a 3-on-3 battle, and Miltank takes all three of his Pokémon out in one Rollout, getting so strong that she deals a One Hit KO to them, and Ash loses the match. This is not only possible in the games, it's downright probable.
 * In Gen II's remake, Bugsy can qualify for this if you're not thinking. In the remake they gave Scyther U-Turn, which takes Scyther off the field to protect him from retaliation, instead. The real horror comes from a strategy with Scyther: Once Bugsy's two cocoon Pokémon faint, Scyther has a STAB 70 power Bug-type attack without drawback (and with Scyther's good attack stat spells trouble) and the other attacks get a boost from Technician, Scyther's ability.
 * Bugsy can still be tricky in the original Gen. II games if it gets Fury Cutter running. Fury Cutter is essentially like Rollout (mentioned above for Whitney) for bug-types. Given how Scyther at the second gym is already pretty difficult, it'll probably take a few hits to go down, so it'll have time to get Fury Cutter built up quite a bit.
 * Remember how sending out a Gastly never works? Well, to be more precise, you sending out a Gastly never works. Meet Morty. This guy has Gengar, which is what Gastly eventually becomes when you've actually bothered to train it instead of backtracking, picking it up and hoping its immunity to Normal- and Fighting-type attacks will magically do away with the trouble you're having. Some exact stats for this species of Pokémon: Speed -- don't bother, it'll move first unless you inflict it with paralysis; Special Attack -- you may now start crying. Before you manage to exploit its weaknesses, Gengar will in all likelihood hurl Shadow Balls at you until you die or use Hypnosis to put you to sleep. Then it'll hit you with Dream Eater, one of the most powerful Psychic-type attacks in the game. Refer to that thing's Special Attack stat from earlier and note that half the damage dealt will heal Gengar. Figured you'd switch out? Nope, it knows Mean Look. If that pathetic sleeping wreck over there having its dreams munched on was your only hope of defeating Gengar you'll have to use a Revive. Which at that point of the game is a strategy that will get you bankrupt.
 * Gengar is far worse in the remakes by means of Gen IV's physical special split. Shadow Ball is FAR more hurtful at this point in the game than it has a right to be.
 * And all his Pokémon have Levitate in the remakes. So if you thought you were smart because you remembered that you can get Dig in National Park... Too bad, it doesn't work against him anymore!
 * Clair, the 8th gym leader from Gold/Silver/Crystal/HeartGold/SoulSilver. What makes her bad is The Ace of her team. Gyarados and Dragonair exist only to make you feel like you can win. With 3 of her Pokémon down she brings out Kingdra, a Water/Dragon-type you will grow to hate. You know that Ice attack you used on the Dragonair? Neutral damage. That Electric attack you beat Gyarados with? Neutral damage. The only attacks that are can score a super-effective hit are Dragon attacks... a department in which you're seriously lacking. It gets STAB on Dragon Pulse and Hydro Pump (both powerful moves) and, if it gets a critical hit, its Sniper ability ups the power bonus from double to triple. Then she has the gall to not give you the badge after you survive hell.
 * It's arguably just as bad, if not worse, in GSC. Not because she had three Dragonair, but that she would use Kingdra to spam Hyper Beam. And back in the day, Hyper Beam was codeword for: you're screwed.
 * And she didn't have Gyarados in GSC either, which is a fairly easy kill due to its quad-weakness to Electric attacks.
 * Karen of the Elite Four in HG/SS. Her Houndoom's speed is VERY hard to match. Once it's set up with Nasty Plot, your annihilation is really a matter of choice. Either it'll flinch you with Dark Pulses or roast you with Flamethrowers, both STAB-boosted, guaranteed to slaughter half your team. Afterwards, there's Gengar, which when you finally bring it to within an inch of its life, it uses Destiny Bond.
 * Jasmine in GSC/HGSS has two Magnemite and a Steelix, both of whom have rather high levels for that stage in the game. It doesn't help that Steel has a load of resistances, making her a tricky fight for the unprepared.
 * Her Magnemite aren't too bad, but Steelix certainly is. 35 is fairly high for that point in the game, and ground attacks don't work nearly as well on it as they did on the Magnemites due to its Defense. And speaking of defense... In Heart Gold and Soul Silver, if you started with Cyndaquil and thought that Flame Wheel would be a good move against it, it's a physical move now.
 * Chuck uses many variations of the Focus Punch strategies (some that are used by many competitive battlers) to make him terrifying. His horrifying Primeape in HG/SS will spam Double Team, then Focus Punch you to death. And it's pretty fast as well. His Poliwrath also makes use of these strategies: it can use Hypnosis to leave your Pokémon completely helpless due to sleep, then follow up with wither Mind Reader and Dynamic Punch (Dynamic Punch is powerful and always confuses, but is very innacurate, Mind Reader makes Dynamic Punch always hit) or Focus Punch (Very, VERY powerful more that won't be pulled off if the user is hit, though if you're asleep, it's a non-issue) depending on if you're playing the originals or the remakes.
 * Blue in HeartGold/SoulSilver. Your team will be nerfed by Exeggutor's Trick Room, making the slowest Pokémon move first. The Mighty Glacier aspect of Pokémon like Machamp and Rhydon is thus conveniently removed, allowing them to destroy you. Of course, if you choose to rematch him, he turns out to have gotten a Tyranitar.
 * Remember Sabrina from higher up on the page? Yeah, she doesn't let up for GSC / HGSS either, as her Espeon will wreck you if you let it pull off a Calm Mind. Doesn't help how FAST that thing is, either. And her Alakazam is VERY FAST and LETHAL, being always faster than you easily making OHKOs to your mons. And it has Recover to restore its health. Do you think you are saved if you try to defeat it with a Dark or Steel Pokémon? Continue dreaming, it has Focus Blast, a Special Fighting attack with a power of 120.
 * Dark and Steel at least walled her a bit better in GSC, due to Focus Blast not existing yet and her mons in general not having as many move options available yet. But if you played GSC and then came into Heart Gold or Soul Silver expecting that... Espeon will be a pretty nasty surprise.
 * Bruno of the Elite Four in the remakes has a Machamp with No Guard, which allows it to still hit your mons even if you use a move like Fly or Dig. And it's moveset includes moves like Cross Chop and Stone Edge, which are both pretty powerful and normally innacurate, but no longer need to worry about that due to it's ability. If you can't take Machamp down in one shot, this behemoth will wreck you five ways to Sunday.

Generation III: Ruby, Sapphire and Emerald

 * Brawly is the second gym leader you face in the game, and this guy hits hard. In Ruby and Sapphire, his first Pokemon is a Machop that knows Leer to lower your defenses, Bulk Up to raise its attack and defense, as well as Karate Chop and Seismic Toss, two fairly strong moves for this early in the game. Then Brawly sends out his Makuhita. It may look like a fat Pikachu, but don't underestimate it - this thing packs a wallop. First of all, it knows a move called Knock-Off, which does damage along with, you guessed it, knocking off your Pokemon's held item, making it unusable. It also knows Sand-Attack, which lowers your Pokemon's accuracy and makes your chances of hitting it insanely low. However, Arm Thrust is what you need to really watch out for. It is a move that can attack for up to five turns, and Makuhita spams this move in spades. To top it all off, it also knows Bulk Up, and will gladly increase its stats for you.
 * And in Emerald, his Makuhita knows the super-powerful Vital Throw, along with Reversal, which is a move that returns the same amount of damage that you do it. So get that puppy into the red zone and you are in for a world of pain. Thankfully, Sand Attack and Knock Off were taken out of its roster.
 * Wattson, especially in Emerald. He uses four Pokémon, all of which know Shock Wave, an attack that NEVER misses. Plus, all of his Pokémon get STAB on it, essentially turning it into a Thunderbolt with perfect accuracy. His various Pokémon have moves like Selfdestruct, a sacrificial attack that is five times as powerful than the standard Quick Attack (and that's without the fact that it halves the defense of the target in Gen 3); Rollout, a move that becomes more powerful after every successful hit; Thunder Wave, an ability that quarters a Pokémon's speed and makes it unable to attack half the time; and SonicBoom, a move that ignores-type effectiveness and can destroy any opponent in two or three hits. The fact that his Magneton is resistant to most attacks and his powerful Manectric evolved early, a-la Falkner's Pidgeotto, add insult to injury. God help you if you didn't pick Mudkip as your starter (Mudkip gains a Ground subtyping as it evolves, making the normally nightmarish Wattson a total joke).
 * Flannery, mainly for her Torkoal. You obliterate her Slugmas and are feeling fine. But then comes the Demon-in-a-Half-Shell Torkoal, which promptly melts you with Overheat, Attract (consider yourself lucky if your lead Pokemon is female) and Body Slam. Then Flannery decides you still haven't had enough and uses Sunny Day, so all her attacks are 1.5 times as powerful now. Sure, Overheat causes a sharp stat loss whenever it's used, but this Torkoal is carrying a White Herb, which restores its stats.
 * Norman. This guy is only the fifth gym leader, yet he has two Slakings, which have the highest attack stat of any non-legendary Pokémon up to then and a ton of HP. Even though they can only attack every other turn, they are still capable of KOing a Pokémon in one hit. He also has a Vigoroth, which is less powerful but pretty fast, able to attack before most other Pokémon you probably own, and isn't crippled by Truant. Finally, all three Pokémon come equipped with Facade, an attack that doubles in power if the user is poisoned, burned, or paralyzed. So if you're trying to break him with stuff like Thunder Wave, he'll just throw attacks at you that are only 10 points weaker than a Hyper Beam; and that's not counting the bonus Slaking gets from being the same-type as Facade. Norman is so tough in Ruby and Sapphire that when they updated all the Gym Rosters for Emerald he was the only one to get nerfed -- he still has Vigoroth and a Slaking but the other Slaking is replaced with Linoone and Spinda.
 * What makes him even more difficult is that his Pokemon are the standard 4-5 levels higher than the last Gym Leader, the usual. However, every other Gym Leader in the game is always several brand new routes away, giving you plenty of never-before-fought trainers to train your Pokemon against. Norman? He's waiting back at the 3rd town you went through, before you even had a single badge to your name. No new areas (besides a small stretch of desert), pitifully weak wild Pokemon, and the game expects you to be able to deal with Norman's power level.
 * And his Spinda in Emerald has Teeter Dance. Sure, it's pretty easy to knock out, but if you don't knock it out quickly enough for whatever reason, prepare to knock yourself out due to getting confused.
 * Let's not forget also that his Linoone packs Belly Drum, which will max out it's strength while cutting it's health in half. It basically goes from a decent but non-threatening ComMon to a lightning quick death machine.
 * And there's also Winona in Gen III. Every member of her team knows Aerial Ace, which always hits. Her Altaria knows Earthquake and Dragon Dance. Once she gets off a couple of them, the blue bird becomes nearly untouchable and hits like a train..
 * Tate and Liza in Emerald, which doubles as a very nasty shock to anyone who played Ruby or Sapphire. In Ruby/Sapphire, the only Pokémon the pair had were Solrock and Lunatone which meant that a Surf or two was enough to cream them. In Emerald, you first need to fight through a Xatu (which can either use Confuse Ray on your fighters or Calm Mind to jack up its stats, aside from flat-out attacking with Psychic) and a Claydol (which spams Earthquake and Ancientpower). If you do manage to get through those two, you'd think Solrock and Lunatone would be pieces of cake, right? Nope. It's not uncommon for Xatu to use Sunny Day before biting the dust, so Solrock can either hit with a powered-up Flamethrower or skip the charging turn to attack with SolarBeam Did I mention Claydol has STAB Earthquake, which hits every other Pokémon on the field and which the other three Pokémon on the Leaders' team are all immune to?
 * They're pretty bad in Ruby and Sapphire as well. Lunatone and Solrock, like most Rock types, have strong physical defense, which is not what you usually get from the Psychic types you've been fighting all game. They also have the aforementioned Sunny Day/Flamethrower/Solarbeam combo to kill any Grass or Water types that try to exploit them being Rock types. It's unlikely you'll have a decent offense with Ghost types, and many Dark types in this game are also Grass or Water types, which as stated, won't last long.
 * In Ruby and Sapphire, the last gym leader is Wallace, who is easy save for his Milotic. But if you are playing Emerald...good luck. Wallace is replaced by Juan, and this guy doesn't play around. He starts out with his Luvdisc, which has a lot of Speed and loves to spam confusion (and Attract if your Pokemon is male) - terrible stats apart from Speed don't matter much when your Pokemon is busily killing itself. And that's just the starting point - his other Pokemon will be quite happy to Rain Dance, boosting the power of their Water moves to ridiculous levels. Want to try a strong Electric-type? Forget it, he has Whiscash, who is not only immune to Electric, but hits it hard with STAB Earthquake. And his Sealeo is all too happy to annoy you with the seldom seen but incredibly nasty Encore. Of course, all of this pales in comparison to his trump card: Kingdra. It has the ability Swift Swim, meaning that while it's raining, outspeeding it is nigh impossible, and that's on top of the rain boosting its power. It also uses the obnoxious Chesto-Rest strategy, allowing it to fully heal its HP and remove status at no cost (but only once; of course, after that once, Juan is all too happy to use Max Potions when its HP gets low). Did we mention Kingdra also has Double Team? Have fun with that.
 * Glacia gives players quite a hard time with her Walrein. It knows Sheer Cold, and it will always work. And, in Emerald, her first Sealeo knows Encore! Wanted to do a quick buff to your stats? Whoops! Now you're stuck spamming that move while getting smacked in the face with a blizzard! She is a tough cookie, before you even have to face Drake!
 * You were led to believe Steven only trained Steel types. Wrong! His very first Pokemon, a Skarmory, has Spikes, which deals damage to any Pokemon you switch in, assuming it doesn't fly or levitate. And it has Toxic, a Poison type move that gets worse each turn. Think you'll use a Fire type to incinerate his Steel types? Not happening: five of his Pokemon can counter Fire. To make matters worse, his Claydol knows Light Screen and Reflect, which increases the already high defense of all his Pokemon.
 * Wallace's champion team in Emerald may be an example of this, especially if you played Ruby or Sapphire before hand and were expecting Steven and therefore started with Torchic, not thinking that you'd need to take a Grass-type along. Wailord has a ton of HP, so it'll take a couple hits even if you have super-effective moves, Tentacruel has Toxic, Milotic has Toxic as well as Recover, Ludicolo has Double Team, Leech Seed, and isn't weak to Grass or Electric... Yeah, sure, he doesn't have a variety of types like Steven did, but they made sure his team's moves made up for that.

Generation IV: Diamond, Pearl and Platinum

 * Fantina was tough but manageable in Diamond and Pearl, but she became a nightmare in Platinum. Her first two Pokémon aren't anything special for sure, but her Mismagius is a force to be reckoned with. When she was the fifth gym leader, it was a decently powerful speedy Special Attacker, but due to being demoted to third gym leader in Platinum, it's now a ludicrously powerful Special Sweeper that's bound to murder pretty much anything in it's way. It has Shadow Ball which is incredibly powerful by itself, but even more so due to STAB bonuses, Psybeam which is very likely (Actually, WILL in the computer's case) to confuse you, and Magical Leaf, which never misses. And to top it off, it has Confuse Ray which is guaranteed to confuse you. Mismagius had these exact same moves in the original games, but in Platinum, you fight it earlier which means your likely unevolved or at least not fully evolved Pokémon can't take hits from Mismagius as well.
 * Cyrus. Gyarados used Waterfall! Bam, you're dead. Reload. Gyarados used Earthquake! Bam, you're dead. Gyarados used Ice Fang! Bam, you're dead. In Platinum, his Gyarados has a Quick Claw. So even if you have an Electabuzz, Magneton, or something really fast with an Electric attack, he might get you before you can slam him.
 * And God help you if his last Pokémon is Weavile. Even bringing in a Lucario won't save you, as the thing knows Brick Break. Distortion Cyrus is far, far above anyone you've fought so far level-wise, and is absolutely brutal. In addition, his movesets for his team are quite a few notches above the standard mook variety.
 * Flint. So you're geared up with your Water, Ground, or Rock-type Pokémon, ready to stomp those Fire-types of his. But then you find out that three of his five Pokémon aren't Fire-type. The Infernape was outright terrifying due to it's high level and great coverage (Flare Blitz, Thunderpunch, Earthquake, and Mach Punch). Platinum was much nicer to us about it, though, giving him a full team of Fire-types.
 * He usually starts off with Sunny Day, and then the "fun" really begins. His Infernape's Flare Blitz combined with Sunny Day will pretty much instantly defeat anything that isn't resistant to Fire, or immune to it in the case of Pokémon with the ability Flash Fire, and even then he still knows Earthquake to take care of them.
 * He's no slouch in Platinum either. He has more Fire-type Pokemon than he does in Diamond and Pearl, with would prompt one to use a Water or Ground type. Think again. If his Houndoom or Rapidash manage to use Sunny Day, you're in trouble. Rapidash is pretty damn fast, (especially against a Rock or Ground type), and it WILL use Solarbeam to OHKO the Pokemon you thought would give you an easy win against his team. Even Garchomp, if it gets a critical hit.
 * Let's not forget Lucian, who directly follows Flint in the Elite 4. He's a Psychic-type user, and his team is nasty. Mr. Mime using reflect and light screen will stop your one-shots. Then there's Alakazam, with huge speed and special attack, then there's his Bronzong. Remember that little Bronzor that kicked your butt before? Meet its big brother. Its type combination already limits your options, but then throw in a Levitate Skill to axe the omnipotent Earthquake attack. Oh, and it can throw Earthquake at you too.
 * What makes this battle even worse is that in Diamond and Pearl, not only is Bronzong 4 levels stronger, but the severe lack of Fire types in the Sinnoh region pre-National Dex (the only ones are the Ponyta and Chimchar lines), so if you started with Chimchar, Bronzong can counter that with Psychic for most likely a 0HKO, and Rapidash has relatively sub-par stats save for its speed, so good luck doing noticable damage. Oh, and the aformentioned Earthquake will still be one hell of a problem for you in this as well.
 * The champion battle with Cynthia is by far one of the most difficult fights of the franchise. Her team is very highly leveled even compared to the fight immediately before her, has varied types, and boasts perfect EVs and IVs across the board, meaning that they're far stronger than normal Pokémon you might fight at the same level. She has Spiritomb,which has no weaknesses to any types, you just have to have a mon strong enough to hit it until it dies. And then there's her Garchomp, a monster of a Pokémon that is so utterly overpowered Smogon banned it from its competitive play rules altogether during the 4th gen competitive metagame. (And the fifth gen metagame as well, despite the power creep.)

Generation V: Black and White

 * Gen V follows the tradition of Normal-type Gym Leaders being That One Boss with Lenora. Her Watchog knows Retaliate, a Normal-type move with a very respectable base power of 70. However, if a Pokémon in the user's party faints on the same turn, it doubles to a whopping 140, and that's not even counting STAB. Since Watchog will always be Lenora's second Pokémon, it can Retaliate right after you defeat Herdier, and it can OHKO pretty much anything you throw at it. It doesn't stop there, either. Her Watchog knows Crunch, a very powerful move at that point in the game, and can put your Pokémon to sleep with Hypnosis. Herdier is no pushover either, having stats comparable to Watchog's and knowing Take Down, which hurts a lot coming from a Normal-type.
 * Her successor Cheren is just as bad in the sequels. He's the first gym leader, and he uses Patrat and Lillipup as his Pokémon, so you'd expect him to be easy, right? WRONG. His signature move is Work-Up, which boosts both of his Pokémon's attacking stats. While his Patrat is easy enough to take out, his Lillipup is bulky for this point in the game, so you're pretty much doomed once it starts fighting back after a few Work-Up boosts. And to make things worse, unlike Lenora who you might at least be able to handle easier with Fighting moves, there is no way to have a Fighting type move at the point of the game Cheren is in, even if you have a Riolu!
 * We also have the fourth Gym Leader, Elesa. She uses two Emolga (Electric/Flying) and one Zebstrika (Electric). Her two Emolga are immune to Ground and know Double Team, and literally nothing will be super effective against them because you won't have any super effective moves for Emolga (unless you caught a Roggenrola, which is very slow) at that point in the game — even worse, one of the trainers you meet before her also has an Emolga. Then her Zebstrika is so fast and so strong it mows down anything that comes in its path. And all three get Volt Switch, which does reasonable damage and switches them out against anything that isn't Ground.
 * And in Black and White 2's challenge mode, she now carries a Joltik with Energy Ball, which will murder any Ground types you'd try to bring in.
 * Elesa even has the dubious honor of coming after another That One Boss: N. Or more specifically, his Sigilyph, which has insane stats for that section of the game - its Speed is high enough to nearly guarantee it the first strike against anything you throw at it, and it knows Tailwind to eliminate the "nearly". Its Special Attack is high enough to virtually guarantee a One-Hit Kill on pretty much any Pokemon that doesn't resist both its Psybeam AND its Air Cutter. Think trying to resist both Electric and Flying is a nightmare? Trying to resist Psychic and Flying may well be even harder. Think it's a Glass Cannon? Not at that point in the game it isn't. Want to try wearing it down with Poison? Don't bother, it has Magic Guard. Burn doesn't work either, as all its attacks are Special.
 * Even within the Elite Four, Marshal can be pretty difficult, because all his Fighting-types have very high attack. His Sawk has Sturdy, which keeps you from OHKOing it. Throh has big HP and will likely not be taken down without you receiving a big hit in return. And in the rematch, he has a Breloom with Spore.
 * Shauntal can be a pain considering most of her Pokemon have a secondary type, meaning that they're actually somewhat well-balanced in comparison to other Elite Four members.
 * If your Pokémon happen to be kinda slow, Clay's Excadrill will likely wipe out your entire team. Made worse by the use of Hone Claws which increases Attack and Accuracy. It's worth noting that your Rival Hugh in the sequels will specifically warn you about the upcoming battle.
 * The Final Boss,, can be a real hair-puller, coming immediately after another difficult fight without so much as a chance to save - thankfully your Pokémon are healed fully between the fights, but this is still a trick the franchise rarely pulls. Some of his Pokémon are more deadly than others; Bouffalant can rip through whole teams with a powerful STAB move (and has amazing defenses), Eelektross is an Electric Pokémon with Levitate (which means no weaknesses) and has the offensive stats to do more than stall. But that's just preparing you for his worst: an underleveled Hydreigon. This thing is monstrous. Aside from the fact that it's almost certainly a couple levels above you, it has astonishingly good type coverage. Thought you'd throw that legendary you just got at it? Guess again, it'll massacre it with STABed Dragon Pulses. Okay, so you'll try Ice. Now he's either melting it with Fire Blast or shattering it with Focus Blast. Finally, if you can whittle it down,   tends to use a Full Restore right before you can finish it off. Despair Event Horizon is right through that door.
 * This is made even worse by the fact that it has been found that the EV's and IV's of Hydreigon are actually maxed, meaning that the stats for that specific Hydreigon cannot go any higher at the level it's at without in-battle buffs. This means that Hydreigon can decimate Pokemon up to ten levels higher than it since its stats are far beyond standard for its level, making it one of the toughest bosses in the franchise.
 * In the sequels, Ghetsis is actually MUCH easier than he was in the original games, surprisingly. However, there's still an incredibly hard boss among the ranks of Team Plasma in the form of Colress. He uses a Magneton, an already solid special attacker that holds an Eviolite, which makes it ludicrously bulky. His Magneton is also pretty bad due to carrying Thunder Wave, and both Pokémon can NOT be taken down in one move thanks to their Sturdy ability. His team also has Pokémon among it's ranks such as a Calm Minding Beeheyem and a Gear Grinding Klinklang. All in all, Colress is one tough son of a bitch to take down.
 * Champion Iris in the sequels is also no slouch either. If you were disappointed by Ghetsis' pathetic underleveled Hydreigon, no worries, she also has an underleveled Hydreigon that's much more of a threat. Like the one Ghetsis used in the original games, it has fantastic type coverage as well as Charge Beam, which boosts it's Special Attack to high levels as well as doing damage. To make things worse, it's her first Pokemon, meaning that she's more than capable of sweeping your team right out the door. And if you do take it out, the rest of her team is quite competent. Her Lapras is a bulky Pokémon with a range of attacking moves, her Aggron is physically tanky, her Druddigon is hard hitting and bulky, and her Archeops is both ridiculously powerful and lightning quick to boot (Unless it's health gets below half, thank GOD for the Defeatist ability). Her Haxorus however is in a league of it's own. It already has a ridiculous Attack stat, but it also packs Dragon Dance which makes it faster AND even more powerful! If it even gets one Dragon Dance off, you're already dead.

Generation VI: X and Y

 * While X and Y are notorious for being among the easiest games in the series, they still pack their fair share of tough bosses, the first one being Gym Leader Grant. He uses Rock types, so like Brock and the other Rock type gym leaders, he'd be easy right? Wrong. DEAD wrong. His Amaura is a very bulky little Pokémon that has the refrigerate ability, which turns Normal moves into Ice ones, meaning he now gets STAB off of the already powerful Take Down, and it will paralyze your Pokémon with Thunder Wave. Then, there's his Tyrunt, which has a Dragon subtyping that will remove a few of Rock's common weaknesses. It's also very powerful in general, as it has STAB Rock Tomb to do decent damage and slow you down, and Bite (Made even more powerful due to Strong Jaw) and Stomp, which will proceed to flinch you to hell if your speed drops below Tyrunt's. Needless to say, Grant almost more than makes up for the amount of very easy Gym Leaders by himself.
 * Siebold of the Elite Four is often considered by many to be the hardest opponent in the game, and for good reason. His Water type team is full of incredibly good Pokémon, with Gyarados being a stand-out example, as it can become a speedy sweeper thanks to Dragon Dance, and it has Earthquake and Ice Fang to eliminate Water's common weaknesses. And even with that one out of the way, his other teammates don't let up. His Clawitzer has a lot of moves that are boosted by it's Mega Launcher ability, Starmie is an incredibly fast Special sweeper packing Dazzling Gleam for Dark types, and his Barbaracle is an all around good Physical attacker.
 * Fellow Elite Four member Wikstrom is also pretty nasty. His Probopass packs lots of pretty decent moves and is impossible to drop in one hit thanks to sturdy, his Scizor packs the dreaded STAB Bullet Punch + Technician combo utilized by lots of competitive players, his Klefki WILL annoy with priority Spikes and Torment thanks to Prankster, and his Aegislash will either tank you completely, or utterly annihilate you thanks to it's stance change gimmick.
 * The final battle with Lysandre is also fittingly tough. His Pokémon are all-around decent with high levels and great stats, but his Gyarados... or shall I say MEGA GYARADOS is more than enough to completely wipe you out. When his Gyarados Mega evolves, it gets a massive attack boost, and it has great type coverage to wipe you out with. It has Waterfall for STAB, Outrage to serve as a simply powerful move, and Earthquake and Iron Head in order to cover it's weaknesses to Electric and Fairy respectively (It gains a Dark subtyping upon mega-evolving, which would make Fairy a weakness... or not thanks to the aforementioned Iron Head).

Pokémon Ranger

 * Charizard in the first game. The second time with him is a lot harder. Not only does he like to fly around while blowing flames, but his fireballs scatter much more densely, making it hard to circle him -- and even with assists, it takes a lot of loops to get him. It also doesn't help that if you take too long, he flees the battle and you have to start all over again...
 * Several players have a pretty rough time with Steelix, too. Even when he coils up (which is about the only time that circling him is even possible), his sheer size makes drawing those loops awkward. Having multiple Water and/or Fighting assists comes highly recommended.
 * Drapion is a rather irritating boss due to the fact that you face two group captures directly before it. And that you come to a full stop at one part unless you happen to have a Normal-type Poké Assist ready.
 * Flygon. You're not allowed any other Pokémon because it's a "test" and the bloody thing resists Plusle/Minun's assists. And it's incredibly fast, making it nigh-impossible to get the amount of circles you need to finish the battle.
 * Entei. Not only is the whole thing a Sequential Boss, you have to deal with the fact that it basically attacks the entire screen. And in the refight, you can only bring three Pokémon for the whole thing...
 * Metagross can be a real pain if you don't have the right assists. It pretty much fills the screen with beams and makes shields for itself.
 * in the second and third games causes damage just by WALKING. It's easy to screw up and lose and assisting Pokémon, and your Styler gets damaged if you don't circle the thing immediately before or after it launches an attack.
 * Ditto in Guardian Signs. You thought fighting each of the beasts individually was bad, this thing will transform into each of them over the course of the fight. Unless you have a variety of Poké Assists, you're in for a long fight.
 * Feraligatr in the third game. It's not so bad before and after it rages, but while it's raging, it tackles all. the. time. And 9 times out of 10, it will charge off screen so you can't circle it between attacks, not that there's much time in between for circling it. And each tackle does 10 points of damage.
 * Salamence in the first game. Like with Flygon, you aren't allowed to have other Pokemon with you. Unlike Flygon, however, it's slow but more than makes up for it by having to draw a ridiculous amount of circles around it. It also outright ignores your starter's assist, and likes to fly above the battlefield for long stretches of time, meaning you can't circle it. Simply landing counts as an attack, and it hurts.

Other Spinoffs

 * Pokémon Colosseum's Cipher Admins; this isn't so much because they were difficult to defeat but because you had to spend ages trying to capture the legendary beasts while keeping yours alive. Especially Ein... Despite being nearly ten levels up, Raikou's Thunderdance combo hurts. He's also a big fan of inflicting the Confusion status, and as is usual for the Pokémon games when you face a boss (like Ein) your team will suddenly become about 250% more susceptible to the detrimental effects of status ailments.
 * Dakim was far more annoying as you had to try to capture the legendary Fire Pokémon Entei, who was weak to Ground moves. Guess what Dakim's favorite move was? That's right, Earthquake! Every one of his Pokémon had it, and got STAB bonus from it as it hit everyone else on the field, which meant that if Entei came out too early, it was going to get knocked out before you could catch it. If their constant Earthquakes didn't defeat you, the high probability of Entei fainting too soon would probably have you replaying this fight over and over again.
 * Don't forget the fact that, when you catch a Shadow Pokémon, all they have is Shadow Rush (at least in Colosseum, anyway)... but before you catch them, their original trainers can use all of their other moves. Hence allowing the aforementioned Thunderdance combo. It also means that there's a chance that your Shadow Pokemon-owning opponent could use Shadow Rush, which gives recoil damage. This means that someone trying to catch a Shadow Pokemon had to be very careful about the opponent's HP; leaving it at 1 HP or near it could leave it vulnerable to its own recoil. At least the Shadow Pokemon owners generally didn't use Shadow Rush, but that doesn't mean the possibility was gone.
 * Miror B, also because he has several Ludicolo that have Rain Dance to activate their abilities Rain Dish and Swift Swim which will either regenerate their health or make them faster than you. Not to mention the fact that they are all fully evolved Pokémon that are at a decent level and have very few weaknesses you can exploit at that point in the game.
 * The final boss, Evice, is pretty difficult, but generally par for the last boss of a Pokémon game. However, should his Slaking and Slowking end up on the field together at any point, you're screwed. Slaking's stats are equal to those of Groudon's. Slowking will use Skill Swap at the first possible chance, which gives it Truant and gives Slaking the ability to attack every turn. Say goodbye to any hope of beating it now. Even if you beat his two, he's still incredibly tough due to the fact that all of his Pokémon are bulky hard-hitters who will boost their attack power even higher. Of note are his Scizor who will Baton Pass Swords Dances to his other teammates, and his Salamence which is just ludicrously powerful in general.
 * The second fight with Snattle in Pokémon XD: Gale of Darkness also hurts quite a bit if you don't have the right number (or quality) of sweepers. Why? Starmie. It hits fast, hard, and if you haven't been dragging a tank of a Shadow Pokémon around to soak up the damage, it can wipe out an entire non-Shadow team. Its Shadow Solrock partner, by contrast, is almost insultingly easy after that thing.
 * And even when you win, Cipher Peon Kleef proves Dangerously Genre Savvy by ambushing your team as you go through the Door to Before, while the next healing machine is through him or another boss and the last one is a major backtrack. Thankfully Kleef is otherwise pathetic (a team of stallers with no Shadow Pokémon to snag) and XD allows saving anywhere, but it really hurts when your team has such major injuries.
 * Thug Zook, when encountered outside the Key Lair immediately after, is a nightmare. He has a Shadow Zangoose with him that is above the typical level for that point in the game, and it will happily kill off one of your team members every single turn, first, without fail. Essentially this forces you to constantly pick away at it with the survivor, hoping you have enough firepower to kill it before it wipes out your whole team, and then proceed to spam Revives and healing items while his other Pokémon are busy trying to kill you less effectively. It's so bad that in the rematch with him, his mons haven't leveled up at all, and he's still a very hard opponent to beat.
 * Grand Master  can be this for the unprepared. Three Shadow Legendary Pokemon (the legendary birds) at Lv. 50+, plus another three relatively strong Shadow Pokemon (Exeggutor, Tauros, Rhydon) at Lv. 46+. What this means is that powerful attacks will be headed your way, and every attack against you will be super effective (Shadow attacks are super effective against non-Shadow Pokemon, but thankfully/possibly no same attack type bonus), as well as a low catch rate for the Legendary Pokemon. If you're not prepared, you'll run out of Revives and/or Snag Balls before all six can be snagged.
 * His right hand man is also incredibly difficult. He has four Shadow Pokémon, the highest amount you will have fought in the game at that point, and they're all quite nasty. Special mention goes to his Marowak, who hits like an elephant thanks to it's Thick Club held item, and his Salamence, an incredibly powerful Pokémon that is at level fifty at a point in the game where you've been fighting Pokémon whose levels are, at the highest, in the mid-forites. An save for his Shadow Manectric, his Shadow Pokémon have a decent amount of bulk which makes whittling them down to capture even harder to do since you've got to deal with a never-ending volley of powerful, always super-effective attacks from his end.
 * Those bloody group bosses in Pokémon Mystery Dungeon: Explorers of Time/Darkness. I'm talking to you, Luxio/Electrike tribe,, 'The Grand Master of All Things Bad' and their cronies. And let's not forget.
 * Primal Dialga is no pushover either. He packs a lot of HP. He's got a great defensive typing (Steel/Dragon). He's got the intimidator IQ skill, which more often than not prevents you from hitting him at melee range. And to top it off, he's got Roar of Time, which can hit both you and your partners from anywhere in the arena, is surprisingly accurate and deals triple digit damage... at a stage when you just reached triple digit HP, which means you'll really need those oran berries and reviver seeds. Oh, and you fight in your unevolved forms and losing forces you to climb Temporal Tower again?
 * Pokémon Stadium 2 brings us Janine. She's easy in Round 1, but what's her strategy for Round 2? Baton Passing multiple layers of Double Team (a move normally banned in Smogon competitive play for being too luck-based). All her Baton Pass targets can take a few good hits, have Confuse Ray, Attract, or Swagger to screw with your chances of hitting even further, and will wear you down with Toxic and Sandstorm. If you're planning to just switch out, take note that she's also packing Spikes and Mean Look. She will slowly torture you to death unless you come prepared with Haze and Heal Bell, but even then you're still at the mercy of many elements of luck.
 * Erika had a really annoying rematch Vaporeon because of the former ruling that Sleep Talk could pick Rest again to restore the health. Vaporeon having a ton of HP and decent special defenses did not help matters at all.
 * Sashay from the "little cup" Sunny Park Colosseum in Pokémon Battle Revolution. Especially her Treecko. That stupid thing spams Energy Ball like there's no tomorrow. A big attack like that on a little baby is a big surprise for those who aren't prepared!
 * In Pokémon Battle Revolution, everyone becomes That One Boss on the "Level 50 -- Rank 8" setting. The boss of the Colosseum will throw uber legendaries at you just to screw you over. "Colosseum Leader Taylor sent out KYOGRE!"--cue rage quit--And it's not just the bosses! On rank 8, everyone leading up to the boss will use 3 legendaries at the very least. And don't even get started on Mysterial!