Lu Bu in the Hu Lao Gate stage of any Dynasty Warriors series is a textbook example of this trope—particularly when he powers up mid-fight. It's the only guy where the suggestion of "run away" is a good one.
Lu Bu in the Xia Pi stage of Dynasty Warriors 4 if you kill Diao Chan. This will enrage Lu Bu, who will power up into "hyper mode." If you want to kill Diao Chan, it's suggested that you weaken Lu Bu to a sliver of life before killing Diao Chan, and then have all of your generals dog pile Lu Bu. (Allied officers whose AI isn't set to "move to location" as top priority will tend to dogpile the enemy commander in a Warriors game.) In general this rule applies, although this troper thinks that Warriors Orochi 2 may have averted it at least once.
On a related note, Wei's Act 3 qualifies as That One Act precisely because you have to beat Lu Bu at some point (if he flees at Xia Pi, he'll face you in the very next battle). If you can do that, nothing Liu Bei or Sun Jian can throw at you should faze you.
Then there's Lu Bu in Dynasty Warriors 5, which added a fun little feature called Musou Rage. Normally a player-only power-up which made you nigh-unbeatable while using it, Lu Bu could also use it. And while you have to find a Musou Token to use yours once, Lu Bu can use his repeatedly. Combine that with a few attacks that are probably impossible to block, and he will turn you into a bloody smear on the wall if you stand toe-to-toe with him while he's in a Musou Rage.
At least the games (particularly Dynasty Warriors 6) give you the chance to put the shoe on the other foot - by playing as him in his own campaign. Where you proceed to utterly destroy EVERYONE. Twice. Including the people who were already dead. That One Boss is a lot more fun when you get to be him.
Dynasty Warrior 6 Lu Bu is actually not exceptionally hard by himself - he has the biggest stats, but merely adequate moveset, and is no longer a top tier character. The problem is, he's always in hyper mode, when you fight him.
In the same vein, Honda Tadakatsu and Maeda Keiji in Samurai Warriors 2. Sure, the computer-controlled ones have a slow wind-up speed to most of their attacks, but heaven help you if you get caught in even ONE of these attacks. There is a reason that in every story battle you come across them, your commanding officer strongly "suggests" you keep well away.
Raoh in Fist of the North Star: Ken's Rage. The final battle with him being the worst, where he gains the ability to use Musou Tensei and become intangible, forcing you to do the same so you can hit him at all.
And yet again for Dynasty Warriors-based games (sure is popular huh?): The first Blueno fight in the main story mode of One Piece Pirate Warriors. True to the anime, This is a Heads I Win, Tails You Lose fight in a cramped space after the already aggravating Galley-La Company level. Like most bosses he has three phases. Phase one has him using Soru to fake a guard and punch your lights across the room, a normal combo, and Geppo to dance around a small space and slam you with an AOE attack. Phase two, he ditches Geppo and that normal combo in favor of Rankyaku, the razor kick. This wouldn't seem so bad, if he didn't fire it almost any time you try to come close, and its insane horizontal range and MORE insane speed makes it very tough to dodge. Then comes phase 3, where he shows off his Devil Fruit ability and teleports, where if you find him... Rankyaku! Did we mention that he adoresthat teleport?! It may be the first Game Over you get in this game!
And following that is the Rob Lucci battle. Fast and powerful attacks, annoyingly quick speed for the big target he is And in phase 2... you start out in Gear 3rd recovery mode with a load of Marines hounding you. And then there's phase 3, which, while you're in permanent Gear 2nd mode, you have to chase Lucci onto a Marine ship by redirecting cannonballs with Jet Balloon (hard in itself) while dodging his constant Rankyaku spamming in the background. Then win a QTE that leaves him vulnerable if you win... but requires you to hit buttons at a nearly impossible speed for the short time they arrive. Did we mention you have to do that whole cannonball into QTE thing twice!? Watching the ensuing Jet Gatling after beating him makes you feel so good after all this hell!
And the NEXT level has Kizaru... yeah... that Kizaru... to start off, every time you "beat" him, you have to go to another area on the map to give chase. Doesn't seem so tough, but he's hounding your crewmates during this... the same ones that you have to protect only for Kuma to blast off, nigh you fail the mission. And he has three phases himself. Phase one, all he has is a simple kick, kick, sword combo, a sword swing with godly range, and his usual lightspeed kick. Phase two, after you meet up with him again, he uses all these with a teleport into laser that's not only hard to see coming, but hard to dodge even if you DO see it coming! Then there's phase 3, after you meet up for the last time, he combines all these tactics with a nigh unavoidable attack where he rams into whoever's in his almost impossibly large horizontal range several times for huge damage as well as an attack where he spins like a maniac while firing a metric crapton of laser beams! And to top it off, unlike most of the games Logia types (and Magellan), he likes to keep himself invincible as long as possible during all three phases, making trying to spam your specials an effort in futility.
Shtrom Jr. in Captain Commando. Sure, the game is crawling with harder bosses, but he's VERY fast, dashes from one side to another of the room in a second and attacks you with flying kicks and an harpoon thrower, mowing your energy down very easily and traumatizing many children who couldn't make it past the second level. The best part? The sixth level boss were A COUPLE OF THE SAME BOSS!
Slice, the fourth stage boss in Capcom's Cadillacs and DinosaursArcade Game. He can blaze about the screen a-la Rolento, and he has ridiculous range and priority with his boomerang attacks... especially the multi-boomerang one which covers half the screen. To top it off, the seventh boss, Slisaurs, are mutated versions of that SAME DAMN BOSS! (three if you're playing a two/three player game)
There is however a simple trick to beat him. Namely precise spamming of the dash attack.
Even that's nothing compared to Tyrog, or as I like to call him, "Mr. Overkill". To recap: 1. Quick-moving, can hit you from just about everywhere 2. Constantly hopping, so nearly every hit knocks him down, making it virtually impossible to do any real damage. 3. Helper mooks constantly rushing in as fast as you can put them down. 4. And of course, you have to beat him THREE TIMES, each time being able to take more damage. Heck, compared to him, Fessenden is a light sparring session!
Fire Leo in Viewtiful Joe - insanely fast, massively strong, on fire, shoots deadly fireballs, and can take off four hearts in one attack. Oh, and he appears at the end of a Boss Rush. On higher difficulty settings you don't even get notification on where your enemy is attacking so it is harder to dodge them. Most common enemies give away other visual cues to make dodging them just as easy (like the common Mecha-Mooks bounce on their feet if they intend to deliver a low sweep kick), but not Fire Leo; the only clues he gives are audio ones. In other words, you can only guess where he is going to attack you during his tornado attack by identifying the sounds and grunts he makes. "Damn you, Fire Leo!"
Another Joe's Mach Speed duplicates, which on easier difficulties are passable chip damage, are a blender of pure pain with the 8x damage in Ultra-V Rated mode.
And for more fun, the only way to damage him involves a hidden property of one of your powers: if you attack an object enough times using Mach Speed, Joe will catch fire, which renders him able to attack the burning Leo. The game never mentions this property.
The worst part is that earlier in the game, Mach Speed is used to PUT OUT fires. Contradictions much?
At least the game makes use of this property for a few puzzles, so anyone who played up to that point knows about it. Of course, if you fail to figure that out, then you beat him with an even better hidden tactic. A zoomed in Red Hot Kick (a skippable upgrade) turns into a fiery dragon kick which beats the barrier. Of course, it amounts to Scratch Damage against the guy, so fighting him this way takes a long time.
Know what's worse? Even if you do manage to get him close to death, he gets even faster.
It's even worse when you get to the fifth level and realize that you have to fight a Boss Rush where all of the bosses you previously defeated come back as That One Boss.
Many of the bosses are like this in God Hand (which happens to be Nintendo Hard), but the Mad Midget Five are particularly brutal. Being outnumbered in a beat-em-up is bad enough, but these little nightmares love to team up, smacking you back and forth between two of them. And since they are midgets, your combo ability against them is limited because any hit to a midget juggles them. They are also one of the few bosses with projectile attacks, adding to the nightmare. Taking them out with the God Hand and God Reels is a difficult exercise; imagine doing this in the "Kick Me" Self-Imposed Challenge. The best part: you fight them twice!
And if you thought that was bad, take a look at the "Three Evil Stooges" boss fight. One has a whip that she loves to use to grab you from anywhere, one has a giant boomerang that once thrown, remains spinning around causing havoc for way too long, and the third just hurts like hell. Now imagine fighting them... ALL AT ONCE.
And then we have Demon Elvis. What better way to end a Scrappy Level than with a boss that will make you weep blood?
The Great Sensei. Insane combo attacks, most of which stun you, huge reach, high health, and the ability to duplicate himself up to seven copies make him a nightmare to take down. Not to mention he is preceded by an infamous Marathon Level that most likely leaves the player with no powerups and low on health.
There's actually a trick, involving the overhead attacks, that lets you lock Great Sensei into a stunloop. Even with this trick, he still takes at least four tries to kill.
Ravel and Debussy. It's bad enough that they are a Dual Boss, but they also have projectile spam and Debussy loves low kicks that you cannot simply duck under. In the Fighting Ring rematch, the second guitarist is likely to show up before you can take down either one, leading to even more projectile spam.
Teenage Mutant Ninja Turtles The Manhattan Project had Leatherhead. He could grab your character and proceed to chomp his face, leading to one of the most damaging attacks in the entire game. He also has a tail whip when you're in melee range. He also has a gun which shoots three bullets which knock you back. His range on his ground attacks was such that drop kicking was just about the only way to realistically hit him, but if he was in his dash this would simply knock you back. Oh, and he and Groundchuck are the only two fights in the game that have a water pit.
It's easier to deal with Leatherhead there than Slash in Teenage Mutant Ninja Turtles: Turtles in Time. He's exclusive to the SNES version and you fight him in place of the Cement monster which appeared in the Arcade version. He has five bars of health, fully blocks all frontal attacks, and he outranges Donatello, making your own survival to the point of when he jumps and exposes his back a chore.
Then again in co-op he easily becomes ridiculously simple to beat. Who can say SLASH PING PONG?
Also the two person battles in the case of Tokka and Rahzar, considering that Tokka can freeze you and Rahzar can burn you. Also the boss fight with Bebop and Rocksteady considering that Bebop's whip attack and Rocksteady's thrust attack both had insane priority. Good luck with these battles if you're playing one player mode considering that even with two players these fights could be very tough.
Streets of Rage for the Sega Genesis has the twins Mona and Lisa. In the first game, they're palette swap clones of Blaze, and behave more like the player characters than any other boss. They're fast, agile, and hit hard, making them very dangerous. The fire-breathing fat guys are a much cheaper version of a That One Boss. When they're charging, they can only be attacked from behind, making it difficult to get at them without getting flattened. When they're not charging, they hide around the edges of the screen, usually making them impossible to attack. They're also the only bosses that can't be thrown or suplexed.
Oddly, a way to (fairly) easily beat those twins is to be really slow about it and cautious. But then time limit! So you lose at least one life anyway. It's also telling that in the third game they come back, and have many more moves, and are still far, FAR easier.
R. Bear from the same game is the hardest boss in the game, hands down.
Abadede was much harder, as R. Bear was vulnerable to quick grabs from behind (you can't hold on long but long enough to act.) Abadede couldn't be grabbed long enough to do anything with, and his short range punches had priority over pretty much everything else, going as far as to not take damage from your attacks as they hit him, even if he hadn't hit you yet.
Madness. Abadede is one of the easier bosses, you just wait until he runs at you, Grand Upper, rinse and repeat ad infinitum, no damage. R. Bear conversely can only really be hurt when he attempts to attack you first, and you have to get in at that split second before his attack lands. Unlike Abadede, you can't see the attack coming from across the screen, not to mention on the harder difficulties there's a Firebreathing Fat Guy and an endlessly-spawning Galsia during the R. Bear fight just to piss you off and get cheap hits.
If you're playing as Max, you can actually catch Abadede with an energy-crushing atomic drop or German suplex if you can get behind him while he does his Ultimate Warrior-style jumping splash. A few of those coupled with a baseball slide to break his charge, and you're home free. R. Bear, on the other hand, is frustration in pixel form.
Jet in the third game is a total pain in the ass. With 4 bars of health (and that's only on Normal difficulty, folks) and constantly summoning Mooks who pretty much attack like he does (flying, staying airborne, slamming you if they grab you, etc.), you are guaranteed to lose a few lives to the flying bastards. Jet only has one and a half bars in the Japanese version, making him a LOT easier. In American Hard mode, he has something like 7 bars, and is utterly ridiculous.
Also Shiva, Mr.X's badass Ninja Bodyguard in SoR2. Your windows of opportunity are very small as he has a counter or knockdown to all of your moves. He also has an invincible attack. On the plus side, his song is totally awesome. He's quite easy though if you know the trick. If you do your regular special move just before his sliding attack lands, your invincibility frames guard you and he's left wide open for a Grand Upper, which conveniently sends him flying far back the right distance so he'll try to do the sliding attack again.
Axel breaks Streets of Rage 2. Playing as other characters can be hell with these bosses, especially Max who is really, really slow; Blaze, whose melee attacks can't wear the bosses down quickly; and Skate who just gets constantly intercepted by R Bear.
The Green Goblin from the Spider-Man arcade game. There are three stages to the fight; in the first and third, you fight him hand-to-hand and he's a wimp. During the second stage, however, he rides his glider and goes back and forth across the stage, dropping about twenty pumpkin bombs on you at once. They're damn near impossible to dodge, and on the rare occasion he decides not to bother with bombs, he flies across the stage so fast he's basically a purple and green blur trying to ram you. He eats up more quarters than any other boss in the game, and he's the last boss of the SECOND stage out of FOUR. When you have to fight him again in the fourth stage as part of a mini-boss rush, he's even faster and supported by goons. The final bosses are Dr. Doom and an infinite army of Venom clones, and neither are anywhere as near as hard as the Green Goblin.
All There in the Manual: In the comic series the game is based on, it's explained that the FF are out of town and Spider-Man had to break in to their HQ to get their sonic gun, so the security machines activated (in the comics, the fight with the robots happened off panel.)
Missing Link from Konami's Vendetta. At seemingly random intervals he would roar and spread his arms, damaging the player and interrupting their attack. Not only was he nearly patternless, but he often didn't flinch from normal hits, having something almost like a super armor ability. Also, it goes without saying he could take a punch and beat you down fairly handily. In comparison, the last boss of the game had a machine gun, but even he moved in patterns and would flinch when you hit him. Also, the Missing Link was in an area with almost no weapons, compared to other bosses who at least had weapon-carrying minions.
See also Drigger in the post-apocalyptic Konami beat 'em up Violent Storm. Thankfully there is no Boss Rush as in Crime Fighters and Vendetta, although the final boss being a parody of Tetsuo from AKIRA more than makes up for that. The obviously palette-swapped ninja turtle boss is also aggravatingly fast.
The Magician from Kung Fu Master. HE should have been the last boss. Even if the last boss, Mr. X, didn't have his Achilles' Heel AI bug, he still wouldn't be as evil as The Magician. A non-crouching attack does no damage to The Magician; it makes his head fall off and he teleports away for no damage. He shoots fireballs that do good damage and, if dodged instead of attacked out the air, turn into monsters behind you. Additionally, stage enemies are still rushing you from behind while this fight is happening. ALSO, sometimes he will, without warning, shoot a fireball down at the floor instead of straight ahead when you approach; of course, this fireball will also turn into a monster, often putting a dragon between you and the retreating boss. ADDITIONALLY, THE GAME'S TIME LIMIT IS STILL COUNTING DOWN!
All the projectile users from the classic arcade game Gladiator. Hacking away pieces of an opponent's armor was the fun part of that game. The stupid block-the-projectiles stages in between those fights were just filler, but then for a stage boss you'd fight a fire breathing or arrow shooting opponent with no armor, which basically took the boring part of the game and made it the hardest part of the game.
Inversion: The boss fights were the least insanely and brokenly difficult parts of Battletoads...Oh, except for the Hypno Orb on "Clinger Winger", which if encountered in 2-player mode makes the game Unwinnable! Haha!
At least until the GBA upgrade...Two words, Slick Trick.
Double Dragon Advance, the GBA remake of Double Dragon, had a weapon and/or a conveniently placed Bottomless Pit in almost every boss fight. One that has neither is the second-to-last stage boss, which is also a Boss Rush: five martial artists whom you fight two at a time. The final one of these has all the special abilities of the previous four bosses, which bizarrely means that he has not one but two different methods of creating clones of himself in addition to turning into an unhittable shadow and knocking you over. If you avoid getting killed throughout the rest of the previous level, it's possible to enter the boss fight with a weapon, but carrying a weapon makes it harder to knock enemies into the pits of deadly spikes, which makes the level that much harder.
And following Lu Bu's example in That One Boss-ism (but not the treacherous nature and Berserker status) is Honda Tadakatsu in Samurai Warriors, especially in the second game where he is a regular character, frequently shows up if you oppose the Tokugawa faction and ALWAYS COME with Hyper Mode. All that can be said is... good luck if you ever try to take him down with a low level character, especially those who are not physically strong. (We're looking at you, Oichi.)
He's also like this in Sengoku Basara. The first game, he's notoriously tough, hard to stagger, and deals a wanton of damage. Start praying to God if you ever get caught in his electric orb move. And this is just on the original game, the Macekre'd version Devil Kings had his damage boosted even further, and you're surrounded with grunts that rarely attack, but if they do, they deal damage as strong as him. And if you try to rush after Ieyasu and forget about him? He teleports right next to Ieyasu and you have to deal with BOTH OF THEM.
Then he ups the ante again in Sengoku Basara 2: Heroes in Chosokabe Motochika's story. Not only does he have to be beaten thrice, each apparition of him gets even stronger, to the point that the last incarnation is also HARDER than in the first game.
Vergil in The Warriors. Not only do you just have Cleon and Vermin when you fight him, but the fight is in 3 parts! The first part, you have to keep chucking bricks and bottles at him since you can't reach him while he throws bottles and motolovs at you. Once he runs to the next room, he plays a game of hide and seek in dark areas. When he comes out, he'll try to kick you in the balls which slowly knocks you down to the ground. While he won't attack you while you're down, he's fond of hitting you like this and running away. Not only that, but in the final room where you can fight him normally, Vergil will occasionally "rage out" like you do, making him ungrabbable and has an attack that knocks you down. Oh yeah, and there's flaming falling debris that will put you on fire if you get hit. Luckily, Vergil can also be set aflame.
Chatterbox is also another That One Boss. When you first fight him, he's on a balcony like Vergil is and will throw motolovs at you while his Mooks deal with you. After you pelt him with bottles and skulls enough, he'll come down to fight you himself. Grabbing him just makes him push you down and his regular attack can also knock you down. Not only that, but if you or your allies get hit by the roller coaster, it's instant death. While you can get Chatterbox to die the same way, he will actually dive out of the way if it is coming. When his health gets low, he will be dazed, thus he can die by the car if he is on the tracks. However, if he is left alone for too long while dazed, he will pop some flash to heal himself and he can do this several times! Oh, and if you are going for a high score to unlock bonus content, Chatterbox dying by the roller coaster won't give you the 1000 point bonus boss kill.
The evil ghost Ezerhorden in the second Dungeons and Dragons arcade game, Shadow over Mystara. Considering if you beat the Bonus Boss from the FIRST game you already know how to beat the Bonus Boss of this game AND the final boss, it's Ezerhorden who wrecks your shit. He has a lot of invulnerability frames, and has resistance to everything but lightning. Since he seems to be a mad spirit it's kind of reasonable he ISN'T the final boss though.
Then again, the dragon's biting attacks, instant-kill breath weapon and hard to avoid melee attacks make the man at Fort Runnels ask you three times whether you want to face it, and for good reason. Deimos is also quite an annoying boss, given his constantly teleporting, immunity to magic and high-level spells.
King of the Monsters 2 has quite a few bosses that could qualify for this. Beetle Master can jab you with his stinger tail to poison you, which slows you down for the scant few seconds he needs to grab you again. He even clones himself once for his second phase! Aqua Slug can stun you in one of two ways; with its grab (which freezes you in ice), or by shooting blobs which make you swell up like a balloon. King Famardy is definitely the worst case of this. For starters, before you even fight him, you have to beat all the other bosses AGAIN (which, due to their tendency to knock you down repeatedly, is not very pleasant). Once you finally reach King Famardy himself, you'll be surprised to see that despite being a gigantic lump without legs, he can leap across the screen farther, faster and higher than every other monster, and he can knock you down JUST BY JUMPING TOWARDS YOU. This property is even more dangerous than everything else in his arsenal, including his ability to fire every other boss's projectile attack.
Haman Khan at the end of Judau Ashta's story in Dynasty Warriors: Gundam. Judau's story is 2 levels long, giving you almost no time to grind either the pilot or the Mobile Suit, and Judau switches Mobile Suits just before the final level. This leaves you with a low-leveled pilot, and a MS which will be level 2 at best, and have no equipment by the time you fight Haman. It doesn't help that you have to fight her four times in this mission: the first three are bearable, but the fourth fight doesn't have any Mecha-Mooks that you can use to charge your SP Attack or get armor repairs from, and when you get her down to 25% health, she recovers all her armor and Turns Red.
The Devil Gundam in Dynasty Warriors: Gundam 2. You have to kill the tentacles before you can touch the head, and the head is the only way to kill the boss... unlucky for you, the tentacles shower Frickin' Laser Beams and fall on you more or less the second you touch them. Even more evilly, some missions require you to take this boss on in a really weak mobile suit, or as part of a Boss Rush sequence, or as part of an Escort Mission where naturally the person you need to protect insists on running directly into the laser barrage.
Although there is a Game Breaker available, in the form of the "Small Giant" skill.
Burning Gundam. Pray you do not run into this pain train from hell as the boss of a mission on Hard. It has the ability to make two clones of itself if it decides things are getting too serious, not to mention that it gets its ludicrous attack power increased even further if either Domon Kashu or Master Asia is the pilot, not to mention that all bosses on Hard have a funny habit of initiating their special move the instant you get their health down to a sliver, as you're attacking them, making it impossible to block and near impossible to get away from unless you aren't juggled and have a special attack of your own saved up to counter it.
A few enemies that decided to not just take the cake of difficulty, but also smear it in your face are both versions of Psyco Gundam and its damned to hell explosions killing your allies off like flies. Do not get fans started on any time Athrun and Kira decide they don't like Shinn and double team him? The less said about the Big Zam the better...at least they fixed That One Boss from the first game: Paptimus Scirocco and Haman inside the damn colony laser. While using the underpowered Hyaku Shiki. While probably Level 10. With them in Hyper Mode and unanimously deciding they hate Char far more than they hate each other...
Dynasty Warriors: Gundam 3 fixed the issues with giant enemies: they now lack their super armor and can be attacked normally (this is especially nice with Devil Gundam, since you can attack him anytime without killing the Gundam Heads first). However, they also introduced Knight Gundam, who in certain stages has permanent Super Mode and such ludicrous attack power that he can kill anything in two hits. That's two basic attacks. Fights with him can last mere seconds on these stages.
In Dungeon Fighter Online, many of the bosses past level 30 are made to be tougher, but one boss stands out. Isadora from the GBL Arad Branch dungeon. Out of all the bosses at the end game that This Troper fought, she stands out as That One Boss of this game. Her attacks include a Meteor spell, summoning spirits to fight with her, a move to detonate said spirits for large amounts of damage, a copy of the Mage's Phase shift spell, teleporting close combat moves that inflict Freeze or Blind, in addition to all of this, she's not alone either. She has several of the Mooks of the dungeon with her, plus two of the more annoying enemies in that dungeon as well.
Sajin the Dragon Swordsman can be an absolute nightmare. The final boss of the Tower of Illusion, his DPS is through the roof and into the stratosphere, his attacks cover large amounts of ground in a relatively small room, and he has really nasty combos that can drain well over half of your health in one go. As if that weren't bad enough, there are ten dragons scattered around the room called Freets that have a rather nasty habit of spamming their ridiculously wide-angle flame breath attack to knock you out of your attacks or simply stun you. Needless to say, you have to use your most overpowered attacks to even stand a chance against this bastard. Naturally, you need to beat him once to get your overpowered attacks as the To I is a job advancement objective.
DonJames in the NES port of P.O.W.: Prisoners of War is more deadly, overpowered, and hair-tearing aggravating than every other enemy in the game put together. He moves quickly and is tough to get to, and has both a five-way gun and unlimited grenades, both of which kill you in one hit. He also has the nasty habit of grenading the spot where your next life begins, meaning you lose another life before you can even move! And just for a little bloody, fragmented icing on the cake, he shows up twice.
General Tragg from Teenage Mutant Ninja Turtles: The Arcade Game (at least the NES version). Not only does he have a lot of health, he's the third-to-last boss in the game. He has a long-ranged attack, in addition to a punch attack that hits hard. Often, he tries to hit you into the electric barrier, if you try jump kicking him. Given you'll probably get KO'ed in the fight, and add to the fact there's an electric fence where he tries cornering you into, which also hurts you.
Also from the same game, Krang. Another boss that takes a lot of punishment before being defeated, and he has quite a few attacks prepared, including a missile attack, a long-range kick that will often hit you, and a punch which he doesn't use as often as the other two attacks. This Troper found this boss harder than the final boss himself, mostly because of how much health Krang has before finally submitting, and given the myriad of attacks Krang has.
Shogun, the sixth level's boss. qualifies for this in spades considering his basic attacks out-range your own and also can can counter your jump kicks unless you have perfect timing. It gets even worse when he Turns Red and his head flies off of his body and around the room trying to attack you while his body also tries to attack you at the same time. He is also the boss of That One Level, so have fun dealing with that.
Hookah from Dragonica. It has a powerful OHKO slash with his hook. Not to mention the shoot who tangles you and stops you from moving and the poison shoot. Expect him to be a nightmare when you're facing him alone. In harder stages, he's even worse, because he can summon his minions and you're screwed if you don't have a party or no potions.
Combined with the fact he's one of the first bosses that has never ending mook spawns.
Oda Nobunaga in Sengoku Basara 3. Normally, he isn't hard, but he'll absorb the souls of his fallen army (up to 6 times) and his attacks will do more damage but at the cost of his defense. The last time is the worse, here he summons a demon to surround him, making his attacks more powerful and unblockable.
Chin Taimei from the NES version of Double Dragon and the arcade version of Double Dragon II. This guy is much more resilient than previous opponents, frequently counter-attacks with low kicks, and the first time you encounter him, there's no room to dodge; at least you can throw him off the building if you're skilled. You'll be facing him as a Demonic SpiderMook many more times in the following missions. In II, he's equipped with nunchucks and almost constantly jumpkicks, and has the ability to knock you into the Bottomless Pit this time.
MadWorld has a pair of That One Mini-Boss. The second actually is a pair, the Cyber Slicers, a pair of robots in the main Shout Out level who have enhanced defense and range, attack simultaneously, coordinate with the Mooks, and will stay back if you near an environment hazard, or chase you to the ends of the earth if you don't. But the most universally reviled Mini-Boss is the aptly named Death Blade. This Grim Reaper wannabe can instantly kill you if he touches you with his scythe, which may not be that hard, as his roller blades make him faster than you, and the scythe has pretty good reach. He doesn't coordinate his attacks to the extent of the Cyber Slicers, but it's still just as bad, because if it does happen, the standard zombies will grab you and trigger a weapons clash in which you have to shake yourself free, but the shaking isn't always recognized and you still die. The best approach to killing him is to hit him with an exploding barrel (there are exactly enough in the level to get the achievement for doing this to him several times).
Notably, the Cyber Slicers and Death Blade are the only mini-bosses to not return for the mini-Boss Rush immediately before the final boss.
Asura's Wrath is a relatively easy game game on normal difficulty if you know what you're doing, but even for a final boss Vlitra's Core does a ridiculous amount of damage and attacks that aren't just extremely hard to dodge in a game where dodging can save you greatly but also have counters that flash faster than you can reasonably react, forcing you to simply pound the buttons and hope you hit the right one. When equipping the Mortal gauge, which Nerfs Asura and lowers his health greatly, EVERY big boss fight becomes this, with Yasha being the first to truly make people tear out their hair and break their controllers in frustration. And each difficulty makes the boss fights even harder than the previous one.
Executioners: It's hard to say which bosses of the 9 levels would qualify for this trope. Mega-Midget(s) of level 4 is one, because he can summon a group of Midget-Men to harass you, and has the "Midget Power!" attack, in which he flies like Superman to try to grab you, and as soon as he goes off-screen, he'll try to stomp on you from above. You must dodge his grab and stomp, grab him just before he can punch you, and punch him up to stand a chance. Blade-Rumbler of level 7 is one, because he has a stomp attack he telegraphs by yelling "Let's Rumble!" and you must jump to avoid taking damage. He can throw some bladed weapon that flies pretty fast and pretty weirdly and I don't know if dodging it is even possible. To stand a chance against him, you must put moderate distance between him and you, avoid getting hit by his bladed(?) fist, grab him just before he can punch you, and punch him up!
The Doctor from Shank 2. He's incredibly fast, his attacks usually do major damage, and he's super tough. He can also heal himself. Admittedly, he's not quite as bad if you just spam him with the shotgun, but even then he's still harder than the actual final boss. If you're legitimately playing the game on hard mode, he's a nightmare.
Armstrong from Metal Gear Rising: Revengeance. Every single attack does 25% of your health, he can self-heal and automatically counters all of your attacks while he does so unless you hit a specific weak point, and the only way he can be beaten is through an insanely hard quicktime event. He can hang on at 0.1% of his health for an infinite time unless you get the final QTE perfect.
Catwoman in Batman Returns, who's faster than you, has a longer reach than you, has annoying tendency to constantly backflip (which renders her invulnerable) and camp out over your body after she knocks you down so she can get in a free hit after you get up, and the first time you fight her she has a dashing slash attack that can kill you in two hits. If she's hanging out in one of the corners of the arena on the same side you are when the attack starts, it's pretty easy to dodge, but anywhere else is mostly down to luck whether or not you get hit.