That One Boss

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I can't defeat Air Man.
No matter how I try to dodge his tornadoes, he just kills me again.
And even though I can get behind him
It's no use, I try to fight, but I get blown away in the end.
I shoot as fast as I can,
But I had to go against Air Man's tornado I was helpless again
I'll try again, of course, but this is my plan:
I'll keep my E-Tank safe in reserve just as long as I can!
I Can't Defeat Air Man (Kiwi Kenobi English Version) note 

There are many different categories of bosses, some more memorable than others. There's the Giant Space Flea from Nowhere, there's the Climax Boss, the Final Boss... And then there's this guy.

That One Boss is, well, that one boss that falls victim to poor playtesting or was thrown in by especially cruel developers. You're leveling up normally, plowing through every battle, until you reach this particular battle and suddenly meet a nasty road block. Eventually, you Rage Quit, or you go on the Internet looking for assistance. In the event that you stop by the game's message board, lo and behold, everyone else is having the exact same problem, with entire topics asking "how do I beat That One Boss?". Walkthroughs usually list the worst offenders as "the hardest boss in the game", and eventually, such a beast receives a reputation for being just that freaking hard. A few of them may be awesome, but that still doesn't mean you won't be chewing on the controller in frustration despite it.

Certain types of boss are not usually included in this consideration right off the bat: The Final Boss, Wake-Up Call Boss, and Bonus Boss. People expect the Final Boss to be tough (Well, usually), and the Bonus Boss is usually included solely to be difficult. Players will prepare for the Bonus Boss by overleveling, but for maximum frustration, That One Boss needs only to just show up out of nowhere, preferably having no place in the storyline beforehand.

They usually show up in Role-Playing Games, though they're not exclusive to them. Game designers will sometimes go out of their way to put them in, serving as a bottleneck to make sure you spend a couple hours Level Grinding before you go on to the next area.

Occasionally, the identity of That One Boss varies depending on an individual's playing style - bosses are often designed to have issues with one particular strategy, so one that requires another strategy can seriously hinder a gamer if they're not prepared to switch up their tactics. Please take a second before adding examples to make sure the issue is actually with the boss and not that your tactical decisions were just poor - if it's hard because it requires strategies you wouldn't have thought of then that can qualify, but if it's just because you hadn't mastered regular blocking timings or got the wrong end of the Elemental Rock-Paper-Scissors system then please refrain. However, any easy tactics like Outside-the-Box Tactic disqualify a boss from being That One Boss... unless the 'tactic' itself is impossible to find.

If the game has multiple difficulty levels, maybe the boss is hard on all difficulty levels, not just the hardest one. You might be up against a particular boss on Easy, but the difficulty of the boss is comparable to the average boss on Normal, or worst-case scenario, the boss may as well belong on Hard difficulty.

They are sometimes called the "brick wall boss", for obvious reasons.

Compare Demonic Spiders, which is more of a That One Random Encounter, as well as the Boss in Mook Clothing (we'll leave it up to you to decide which is worse). Most likely to be the one to use That One Attack. See also That One Sidequest and That One Level. Contrast Anti-Climax Boss, where the boss is underpowered/easy despite story reasons. Direct inverse of Breather Boss, where the boss is just pathetically easy unintentionally. For bosses that are not technically difficult, but are downright frustrating, see Goddamned Boss. For bosses that are deliberately impossible to beat, see Hopeless Boss Fight. If you're looking to read up on unfair bosses that the game seems to deliberately give every known advantage to, requiring you to either get by on luck or cheap gameplay, head on over to SNK Boss. That page is for brokenness - this is just plain hard.

To sum it up. If the boss is difficult when you first get to him, but when you get enough skill (or understanding of the needed mechanics), it is easy on further playthroughs it is merely a Wake-Up Call Boss. If it requires a very unusual tactic to win, but dies easily to it, that is Outside-the-Box Tactic. If the boss isn't technically hard to beat, but doing it is such a slog that you still hate doing it, it's Goddamned Boss. It has to be much harder than expected for that point in the game, and remain that way every time you replay the game to qualify as That One Boss. In other words, this is the boss whose battle will haunt your nightmares every time you see their image or hear their name.

It is useful to note that, as its name implies, most videogames only have one That One Boss. If a game appears to have several candidates for the title — especially if every boss battle feels this way — you are probably looking at Easy Levels, Hard Bosses, or a game that is just plain Nintendo Hard. In fact, if anything, a generally easy game will be more vulnerable to this trope, as while hard games will have expectedly hard bosses, it can be a shock to play a comparatively effortless game and come across a boss where you have to be on your toes for once.

NOTE: Final Boss and Wake-Up Call Boss cannot be That One Boss without being overly hard by their standards. Please do not add them as examples. An Early-Bird Boss will almost always qualify as That One Boss as well, but for reasons of the player's weakness rather than the boss's strength, so those bosses are better placed in the Early-Bird Boss article. Bonus Boss is completely banned from this page (though Rhythm Games are handled differently; see its section for why), as they are optional and they are meant to be overpowered. There is no measure for them. See also: That One Level.

Because there are so many examples of this trope, they have been categorized by genre for your viewing convenience.

Miscellaneous Examples

  • Fatal Frame
    • Fatal Frame II: The second battle with the Kiryu Twins. One of them is a possessed doll and will not take any damage from your camera and there's no proper indication which of them is the invulnerable doll and which the spirit you need to actually fight. So you're already hesitant to use your better film here, in fear of wasting it. They also tend to attack from opposite sides, forcing you to relocate and try to get both in one shot. And that's the problem with all of their battles, but the second one is the worst because it takes place in the Doll Room. This room is pretty small, because it's filled with dolls and dollstands, so you have less room to evade them and can easily get jumped.
      • Also from that game, Sae. Majority of the time, she is surrounded by a red mist that makes it very difficult for your camera to focus on and damage her and she only has short moments without the mist, that allow you to properly attack her. The main issue is her apparently random AI and tendency to teleport around; she can easily teleport right in front of you and get a good hit in. Fortunately, she is the only Final Boss without a One-Hit Kill... unless the difficulty is set to Nightmare and the player is forced to do a No Damage Run against her.
    • Fatal Frame III: Reika. Contrary to Kirie and the Kusabi from the first and second game, who had One-Hit Kills but were rather slow and had easy to read movements, Reika is the complete opposite. She likes to flit around the field and fly up into the air, which is already difficult to see on its own and good luck trying to avoid her dive-bombing onto you. Her own One-Hit Kill involves her appearing at some random location near you. And if you aren't looking clearly away from her at this point, she will laugh at your health bar depleteing.
    • Fatal Frame IV: The late battles against the Utsuwa (Vessel) and Kanades (Accompanists). The Utsuwa is on her own, but she likes to teleport frequently and attack randomly. So unless you get the Evade function down pat, she'll be hell to fight. The Kanades are worse, though. There are five of them and they team up well; four of them are likely to remain in your field of vision and distract you long enough, so the fifth one can easily attack you from below.
  • Lone Wolf
    • The Chaos-Master from the 11th book in this series of gamebooks. It's a hideously hard fight if you don't bring the Sommerswerd along. If you do? Chances are you will die horribly. There's a particularly infamous Let's Play of Book 11 that had the players redo the Chaos-Master fight twenty-seven times before they finally won. It's that hard.
    • So hard that pretty much everyone forgot about the series' FIRST killer enemy, the Gnaag Helghast. If you have a decent base Combat Skill, the Sommerswerd, and Psi-screen, it's a fairly challenging fight. If you don't have a decent basic Combat Skill... well, it's immune to Mindblast, so you're looking at a Combat Ratio of +1 under the best of circumstances. If you don't have the Sommerswerd, you can't do double damage. If you don't have Psi-screen you lose 2 Combat Skill points every round.
    • The Vordak fight on the skyship deserves special mention. If you don't have the Sommerswerd, you have a grand total of four rounds to win two fights; fail, and the skyship explodes and you're toast. If your starting Combat Skill is too low, it is IMPOSSIBLE to win this fight. It was so bad that Joe Dever gracious extended the time limit to six rounds in the Mongoose Publishing re-release.
    • Bested the Gnaag Helghast and those annoying Vordaks? Well done, now get ready for Zakhan Kimah! If you have the Dagger of Vashna, you can settle this with a single do-or-die throw. Otherwise, prepare for a combat ratio of at best -3, and he has 40 Endurance. Oh, and if you have the Sommerswerd, he's also immune to psychic attacks.
    • Then there's Darklord Kraagenskul. Have the DoV and a reasonable number of Combat Skill boosts? Somewhat tough, but managable. Don't have it? Prepare to fight crypt spawn, then make a do-or-die lunge for his sword (although if you're successful, Kraagenskul isn't extremely tough). Have the Sommerswerd? You have a time limit to beat the crypt spawn and then have to make a do-or-die sword throw, otherwise you'll get killed even if you defeat Kraagenskul.
    • And then there's Ixiataaga, who comes at you with a Combat Skill of 60, and also at the tail end of a number of challenging battles, including Tagazin.
  • Not actually a game example, but in the manga The World God Only Knows, Keima Katsuragi can capture any girl... any girl... with the exception of Yui Goido. He himself notes she is a serious player, introducing a new meaning to Player vs Player. He eventually wins... by playing the Distressed Damsel.
  • Ashley's boss microgame from WarioWare Touched features borderline Bullet Hell patterns of projectiles to dodge and an end boss that has three separate pieces shooting things at you.
    • In Warioware Twisted, all you need to know are these six words to know you're in trouble: "Ladies and gentlemen: Wario de Mambo!"
      • The same game has Dribble and Spitz's boss stage. You need to outrun a boulder and jump over holes. Simple enough, but how is the outrunning done? By shaking the GBA in a violent manner! This can make it hard to see the holes, and if you can see the holes, then chances are the boulder is on your tail. Oh, and you need to get 5 points on it in the Spindex, but good luck doing that when it goes to level 2 and 3; those difficulties make the boss microgame nearly impossible.
    • In Warioware Inc., (at least in the GBA version) there is the boss stage called "Punch Out." And the third level of that contains an opponent that is fast. Remember when the level up boss stages are optional? Here, this is NOT the case; Remix 2 will not let you through until you beat him, and he might be hard for first time players. He moves the fastest out of all of his opponents, so you need to have perfect reflexes. However, remember the One-Hit Kill move that the second opponent had on one heart? Here, he can use it anytime he wants, so if you are not quick to dodge, and you are on your last life in Remix 2, it's back to start for you.
    • Warioware DIY has Orbulon's boss stage in the form of a slider puzzle. Why is it hard, you may ask? Well, the puzzle is randomized, so you can never get the same puzzle twice. And you need to do this with the timer breathing on your neck, but sometimes you can be stuck on it; should this happen, you're screwed. And what is worse is that for Orbulon's set to be completed (getting all Orbulon's micro games unlocked) you need to do this slider puzzle multiple times.
  • While the final bosses of the Ace Attorney series usually take longer to break than other culprits, the final showdown with Quercus Alba in Investigations gets special notice for being so long and difficult that there's a save point in the middle of the fight. And here's a song about how difficult it is. Amusingly enough, the song in that link is a parody of the Air Man song that is the page quote.
  • SD Gundam Capsule Fighter has three of them.
    • The first is the Psyco Gundam in the "Kill Three Psyco Gundam" mission, due to the fact that, at harder levels, it has infinite special attacks and, thus, it will gladly go into its Beam Spam special to prevent being killed. Even more frustrating when it does so at its last sliver of life with only 10 seconds left in the match.
    • The second is the Apsalus II from the "Destroy the Apsalus II" mission. Not only does it have a stun beam weapon and an ability that reverses your controls temporarily, it's flanked by two Acguy Repairs meaning that if you don't kill them off ASAP, it'll just keep regenerating its HP while it keeps hitting you with beam weaponry.
      • What makes the Acguy Repairs even more of a pain is that they're one of the few units with Auto Lock-On Jammer (which disables Auto Lock-On when the unit reaches 30%), thus you're resorting to the old Eyeball Mk I to shoot these guys down!
    • The third is the Aile Strike Gundam from the "Test Factory" mission. It spends 9/10ths of its time boosting around the boss room in the air and, as you're trying to shoot him down, you're attacked by Balls, Hizacks and Astrays, as well as missiles that will outright stun you in your tracks.
  • Non-video game example: Exalted second edition's Deathlords are hugely overstatted and obnoxiously powerful, to the point where people can wonder in all seriousness why they haven't destroyed the world yet all by themselves. They were nerfed multiple times, most notably when their wtfhax perfect defense that didn't count as a Charm activation was clarified to only work a handful of times before having to be reactivated, but they remained ridiculously powerful.
  • Minecraft:
    • The Ender Dragon falls into this category, especially notable in that it was the only boss before version 1.4 was released. It flies out of sword range, likes to knock the player off of things, usually off of The End and into the void, and regenerates health whenever it goes near an undestroyed Ender Crystal, located on the tops of very tall obsidian pillars.
    • The Wither is an even worse offender. It is capable of killing you and all your friends and allies before the fight even begins by releasing a massive explosion from its body. The actual fight is no better, it flies around and spits exploding skulls that also gives any target it hits a 'Wither' debuff, with said debuff draining their health bars, but unlike normal poison, this debuff will kill the target instead of leaving them with 1 HP, and thanks to the Wither being a three-headed monster, it can spam the skulls as well as attacking multiple targets simultaneously. You can snip it with arrows in the first part of the fight, but after half of its health is gone, it gains a shield that makes it immune to said arrows, leaving you with Splash Potions of Health/Healing and your swords. And if you try to hide? The Wither will eat through blocks you're hiding behind. Did we mention it has a Healing Factor?
  • Nintendo Land's Pikmin Adventure attraction has mostly average boss difficulty... except for the Emperor Pinchipede. For his first phase, he has about 7-9 weak points, some big and some small. When you take out a small weak point, it disappears. But when you take out a big one, it stays on the field, acting as a spinning disc of doom that wanders about and damages you if you touch it (but not the boss). You must take out all weak points to move to the next phase, and it moves too fast to be able to reliably aim at any specific weak point, so you'll most likely hit the big ones and have 4 spinning doom discs on the battlefield while you continue to toss pikmin at the small weak points. For the sake of saving space, I won't go into the other phases, but know that they are both worse.
  • Winnie the Pooh's Home Run Derby has a bunch of these.
    • Piglet throws his balls much faster than Eeyore and Lumpy before him do. It's a major Difficulty Spike.
    • Kanga and Roo start to bounce the balls, which can be frustrating when you're used to the ball coming straight at you.
    • Rabbit throws the ball really slowly, and then the ball comes REALLY fast at you. If you're playing against him the first time, you'll freak out.
    • Owl zigzags the ball and the ball comes in really fast. The zigzagging ball makes it easy to score a bunch of Foul Balls and unless you've upgraded the Power Stat, successful hits will often stay in the park because of the wonky ball momentum.
    • TIGGER. He will throw the ball at you normally, but then IT BECOMES INVISIBLE. Yes, the ball is invisible. And you have to successfully score 28 homeruns out of 40 balls. That means you can only miss, hit, or foul 12 balls.
    • Christopher. Freaking. Robin. You have to hit FORTY HOMERUNS out of FIFTY balls. Christopher Robin can use a combination of ALL of the strategies of the other characters, and will usually use Kanga and Roo's or Owl's technique. And he's capable of using invisible balls too. Oh he can throw balls normally too, but they're usually too fast for you to hit.
  • Technically not a video game example, but Dino Attack RPG had a few infamous bosses that would certainly qualify:
    • Cam O'Cozy- oh boy. Where to start here. The guy must have been defeated at least five or six times before he was finally killed off just because of how much he frustrated everyone. First he apparently fell to his death by riding a bomb, then he returned as a Big Bad plotting to murder anyone with the slightest idealistic beliefs. He was arrested by the cops but (without his creator's permission or involvement) he came back yet again on Adventurers' Island, and was finally given a Robotic Reveal that served to both get rid of him for good and justify his presence in the RPG to begin with.
      • Duke was practically The Dragon to Cam O'Cozy, and just as bad (if not worse). If not for popular demand he would have very easily escaped from a maximum security prison with relative ease.
    • The Brickspiderbot: Simply fighting this thing practically took up an entire story arc (funnily the same arc with Cam O'Cozy and Duke).
  • Warframe:
    • The dual boss fight with Captain Vor and Lieutenant Lech Kril on Phobos isn't that hard, as the bosses don't do much damage, but it is NOTORIOUS for glitching out and being impossible to beat. Have fun farming the Miter, Twin Gremlins, and Trinity!
    • General Sargas Ruk can be a pain in the ass for new players. The first phase of the fight is pretty easy, all he does is spray fire at you, and you have to blow off his arm. The 2nd phase he gets a room clearing Supernova that's next to impossible to dodge and knocks you down, doing a huge chunk of damage, he also gains a missle attack in this form that will absolutely WRECK you if you get hit by it, and you WILL get hit by it. His 3rd phase cuts the bullshit and he gains an attack that launches Fire Pillars out of the ground underneath you similar to Ember's World on Fire, with the appropriate damage, standing still for more than 3 seconds with kill you unless you're playing as Rhino or another beefy frame.
  • In Puyo Pop Fever, normally you have to deal with Popoi at the end of a story route, who is already fairly challenging. However, if you do well enough on the hardest story route, you run into Carbuncle, who will outright BREAK you. Basically, his dropset gives him a ton of large Puyos-more than any other character in the game-making it easier for him to both set up large combos or offset repeatedly to trigger Fever. Couple this with a brutal AI that can set up 5 and 6 chains in the blink of an eye and is prone to immediately going into a 30-second Fever mode the moment you think you have him threatened and you have one of the nastiest single-player fights in the series.
  • Pinball example: In Stern Pinball's High Roller Casino, the Craps game comes across as this, as it has a difficult target to shoot, an open-ended number of shots to make, and can only be advanced during single-ball play.
  • StreetPass Mii Plaza: The final boss of Warrior's Way, Emperor Fynalle. Every other battle up to him was a mix of properly dividing an army and Tactical Rock-Paper-Scissors. Fynalle, on the other hand, is a straight out five round game of Rock Paper Scissors against an army that starts out eight times your size. There's no strategy; it's a Luck-Based Mission that requires either grinding for Play Coins or extreme luck in telling what he will do.
  • In Tony Hawk's Underground, it's the final skate-off with Eric Sparrow. He challenges you to a race across your hometown to regain a tape that had your ultimate stunt on it. To win it, you pretty much have to play Follow The Leader, hitting every marker while Eric drops... some sort of flames to mess you up. Oh, and you only have 3 minutes to do this. Thankfully, the game decides that once is enough and subsequent plays use a version which gives you a very satisfying Take That, Scrappy!.
  • Bloons Monkey City takes this to Tower Defense genre. Can you take on Bloonarius who can spawn a lot more bloons as it degrades? How about Vortex who stuns Bloons in its radius? Or maybe Dreadbloon who can cover itself in ceramic properties every degrade while also having lead properties? Or something like Blastapopoulos who can shoot stunning lava? Didn't we mention they can get even tougher than a Z.O.M.G.?
  • Alone in the Dark: Li Tung in the third game. He moves and attacks much faster than Carnby can, and can close the distance very quickly and jump-kick you from halfway across the room you fight him in. Beating him essentially requires running back and forth, popping off Winchester shots whenever you get the chance.
    • A couple from the fourth game.
      • The Procuraptor in the library can take a lot of damage, which forces you to use the rocket launcher. Even then, you can't damage it in its normal state, but shooting it with any weapon (even the revolver) will cause it to enter a vulnerable state during a few second where you can actually damage it. However, since this is not obvious, you can waste a lot of rockets by using them during the first state. If you didn't find enough rockets, expect a very long fight as you'll run out of them and have to resort to the shotgun.
      • Alan Morton. A brutal Lightning Bruiser with no clear attack pattern (thank goodness he can only attack at close range). And to top it off, he's a Puzzle Boss (your weapons can only knock him out for a while). You have to knock him out and find the right door which will lead you to a magical spear that kills him with one hit. And if you reach a door while he's up, he'll teleport and slam you down. But once you know the right door...
  • The majority of bosses in Terraria have the potential to be this.
    • Skeletron Prime if you're trying to beat him solo. He has four arms, each of which can do a fair amount of damage, and his head gains defense when it spins instead of losing it like the last version did. His high health and arms will soak up damage to near the end of the night, and if you don't beat him by then he becomes practically unbeatable and gets a One-Hit KO attack.
    • Wall of Flesh. If you don't prepare yourself in a ridiculous manner before the fight, you will have a LOT of trouble. First, you have to contend with the Hungry, mouths on tentacles that do ridiculous melee damage and become flying critters when killed, forcing you to kill them twice. For added annoyance, they respawn on Expert. They make ranged weapons a problem to use (unless you use Meteor Bullets or Unholy Arrows). If you get past that, you still have to deal with the Wall itself, which has very high health and uses Beam Spam from both eyes with increasing frequency as you do damage. On top of that, it accelerates as it takes damage; on its last legs, the Wall is actually faster than the player at full sprint. NO strategy will assure success against that freaking wall. Which makes coming back with Hallowed Armor, the Megashark, Crystal Bullets, and Excalibur (or, heaven forbid the Terra Blade) for round 2 (or whatever number fight it is) all the more satisfying. Adding to this is the fact that he's prone to bugs. He may accidentally spawn heading the wrong way, which is pretty much instant death because any prepared player will summon him at the far end of the map. He can also spawn much further down than he's supposed to, making the world-spanning bridge you're basically required to build if you don't want to spend 90% of the fight swimming in lava or jumping over pits and structures useless. There's reason why the characters' page says there's no middle ground - you're either overprepared or dead.
    • The Destroyer has become notorious as the most difficult of the three mech bosses, ever since it got buffed. It was briefly nerfed in 1.2 reducing its defence, damage, and health, but then 1.2.3 came along and reversed the nerf big time, raising its stats even higher than they were before. Additionally, there's less healing for you during the fight (since probes now only drop hearts 33% of the time instead of 100%) and it's completely immune to all debuffs!
    • The Twins are a Dual Boss that can easily box you in with little effort thanks to their huge hitboxes, incredibly fast which makes dodging them very tricky, and their Turns Red mode takes their attacks Up to Eleven. Spazmatism has a cursed flame attack in his panic mode with a ridiculously huge hitbox, and charges at you with barely a second to dodge. Meanwhile, Retinazer's Eye Beams start getting spammed to Bullet Hell levels. The only reliable strategies many players can cook up is "make a giant sky bridge that extends all the way across the world, shoot at Spazmatism while running away on a fast mount", or "use Gravitation potion and keep switching gravity as you run away from the Twins and shoot at Spazmatism".
    • Plantera suffers from a similar problem. She has a ton of HP, shoots petals (homing in Expert) and spiked balls that do a lot more damage, but if you prepared with an arena then her first form is basically a cakewalk. All that changes when she gets down to half health. She goes berserk, doubles in speed (sufficient to outrun a player wearing Lightning Boots and using a Swiftness potion) and causes even more damage. For added measure, Chlorophyte homing bullets will be negated by the Combat Tentacles she spawns for the second half, which forces you into either melee or switching to different bullets and hoping you can get enough shots through. And don't even think of going to the overworld jungle to get the advantage—She absolutely hates it.
    • The Pumpkin Moon. It starts out simple, with Scarecrows, Splinterlings, and Hellhounds that can be fairly easily handled with good crowd control weapons. Then you hit wave 4 and the Mourning Wood shows up. It's got 12,000 HP, it fires a huge spray of fire that sticks around on the ground for a few seconds and does huge damage, and it's surprisingly fast. It only gets worse from there.
    • The Frost Moon. It's The Pumpkin Moon Taken to Eleven The first few enemies like Zombie Elves and Gingerbread men, can be handled with ease with Pumpkin Moon loot, then you hit Wave 4 and the Everscreams come out with 13000 HP, being a harder version of the Mourning Wood. Later, you have to fight Santa-NK1 Which fire very powerful missiles and presents, that has 18000 HP. Even later, from waves 12 onwards you fight the Ice Queen with 34000 HP, which is pretty much Pumpking on crack, flying everywhere, firing ice shards. It gets hectic really fast, good luck getting past wave 12 on your own. To put it? Some people consider the Ice Queen to be tougher than Duke Fishron (see below).
    • Duke Fishron introduced in 1.2.4 version. He's extremely fast, powerful and has 50,000 HP to boot, fighting him in a small-medium sized arena is downright suicidal as he will create huge tornadoes that not only shoot high-speed, high damaging sharks in a very fast rate, but will also limit the player's movements. Think it can't get any worse? It Turns Red when low on HP becoming even faster, 1.5x as painful, and more erratic. In 1.3, he got buffed even further—he will get pissed and now repeatedly use his fast charging attack if he leaves the ocean.
    • In Expert Mode, all the bosses become this, as they're twice as strong while you aren't. This is especially bad in the early game, as your options are limited because the bosses are holding the items you need to progress.
  • Sentinels of the Multiverse has The Chairman. 15 of his 25 cards are minions of one form or another, and the deck's mechanics mean the field will start out heavily populated and the swarm will only grow. The heroes have to defeat a certain number of minions before The Chairman's card flips and he can even be damaged... but each one defeated provokes a high-damage counterattack from The Operative. Even when The Chairman finally does become vulnerable, he counters almost every attack made at him for heavy damage. Even if The Chairman is defeated, the heroes don't win until they can bring down The Operative as well; if this happens, The Chairman goes into the discard pile like any other defeated target... which means if the Villain Deck is reshuffled (and considering he starts with 10/25 cards in the trash and you need to defeat a lot of minions to get this far, it probably will), there's a chance he can be played like any other card, return to the field invulnerable side up with full health.

http://tvtropes.org/pmwiki/pmwiki.php/Main/ThatOneBoss