I can't defeat Air Man.
—I Can't Defeat Air Man (English version)
And then, there's That One Boss, the boss that falls victim to poor playtesting. You're levelling up normally, plowing through every battle, until you reach this particular battle and suddenly meet a plethora of Game Over screens. 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.
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 (usually), and the Bonus Boss is usually included solely to be difficult. However, it's still definitely possible for them to count if they're hard by their standards: sometimes a Final Boss presents a baffling difficulty spike in a game that has otherwise been smooth sailing since the beginning. A "Wake-Up Call" Boss might not even be a "wake-up call", but a cheap kick in the balls that is more unfair than what's to come. And as the SNK Boss shows, some Bonus Bosses can take the intended difficulty of a game's ultimate challenge and make it outright insane.
They usually show up in RPGs, 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. So while Revive Kills Zombie can disqualify a boss from being That One Boss, players either unwilling or unable to use 'revive' can still have a hard time.
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. See also That One Sidequest and That One Level. Contrast Anticlimax Boss, where the boss is underpowered/easy for story reasons (or despite them), and 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.
Because there are so many examples of this trope, they have been categorized by genre for your viewing convenience.
- Atlus (No, this isn't a genre, but a company. Yes, a company makes games with so many sadistically hard bosses that it needs its own section.)
- Action Games
- Beat Em Up
- First Person Shooter
- Shoot Em Up
- Stealth Based Game
- The Chaos-Master from the 11th book in the Lone Wolf 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.
- 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 Damsel in Distress.
- 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.
- Nethack hasn't got many bosses, but of those few that are there, the Wizard of Yendor can be the most mind-numbingly annoying - and dangerous as well. His abilities include casting powerful spells (some of them capable of being instantly lethal) as well as stealing certain important MacGuffins and creating clones of himself. And as if that wasn't enough, he won't stay dead - even if the player kills him or escapes the level, he will periodically cast nasty spells on the player or even return to fight in person again until the player reaches the Astral Plane.
- While the final bosses of the Ace Attorney series usually take longer to break than other culprits, the final showdown in Investigations gets special notice for being so long and difficult that there's a save point in the middle of the fight.
- Warframe has a few examples:
- Counselor Vay Hek, a flying bullet sponge with completely random movements, fully invulnerable except for a tiny weak point that's revealed only when he buffs his soldiers, a machine gun of his own and an unavoidable attack that always applies a magnetic debuff, weakening your shield and draining all your energy. He has a second phase when weakened enough, but taking a different form that's fully damageable and is much easier to hit. The fact that he flies away when he loses rather than dying like other bosses adds insult to injury.
- Mutalist Alad V, invulnerable unless he uses one of his special attacks, leaving shorts openings, not helped by his shield that may fully recharge between two openings and the constant swarm of Infested coming from every direction and hitting hard when they get close. Without some high burst damage to punch through his shield or toxin damage to bypass it, this fight is nearly unwinnable. Also noteworthy is his ability to mind-control players and turn them back on their allies, some warframes being capable of healing him to full or wipe out their entire squad (solo players don't have to worry about this, fortunately).
- The Vagabond in Yu-Gi-Oh! DUEL LINKS. This character's deck is an exact copy of a player's who scored high in the previous month's PVP ranked duels, so he's not only hard to beat, he's almost impossible to prepare for. He may even have cards on the Forbidden List, if it changed over the previous month. Of course, he gives out more XP and rewards if defeated than other opponents.
- While Five Nights at Freddy's: Security Breach doesn't exactly have boss fights in the traditional sense (save for Monty, sort of), your first encounter with the Daycare Attendant's Moon form is a tough (and terrifying) one. You have to run around the pitch-black Superstar Daycare and restore power by finding five generators and turning them on. The problem is that they're scattered around a confusing, maze-like playplace, and Moon is a Super-Persistent Predator who blends in with the darkness of the daycare and can ambush you from surprising angles thanks to his ceiling-mounted wire giving him free range of the place. You can distract him by knocking over stacks of boxes to trigger his Neat Freak tendencies, it doesn't do much to help when you're trying to find that Last Lousy Generator.
- Almost. The one clue you get is the "pickup card", the strongest card in his deck, revealed when you click on him.