Tactical Suicide Boss

"I-have-no-will-to-live-so-here-is-my-weak-spot-please-shoot-it Boss Battles"

—Yahtzee, Zero Punctuation

In video games, it's quite common for a boss to only be vulnerable at a particular time. It's one thing if it's due to the boss needing to catch their breath every so often or the like. But sometimes, it's just because a given attack leaves them open. Maybe it exposes a Weak Spot while preparing the giant laser, maybe their attacks double as weapons or platforms that the player can use to reach their target, maybe on occasion it decides to just stand there laughing at you with its defenses down, or maybe it will keep charging head on into walls until it dies.

This is a specific form of Hoist by His Own Petard - the boss only has to avoid one particular action to be literally invincible. Since it involves the boss intentionally leaving themselves open, it also heavily involves carrying the Villain Ball. Finally, Fridge Logic heavily comes into play - it's usually in retrospect that you realize What an Idiot! the boss in question is for continuing to perform the attack even after it's been exploited several times. With animal bosses or others too unintelligent to realize their mistake, this can be justified, and to an extent it can make sense with robotic bosses as well, but in that case you have to wonder who designed and programmed them...

Hopefully, sooner or later there will be a variant of this where the boss does stop using a particular move that leaves it open to attack... only to then use a different move that also leaves it open to attack.

Bosses like this can occur with any of the three types of AI (as described in Artificial Stupidity)

AI Roulette: by far the most common for this trope. It will use one of several attacks randomly, but only a fraction of them (often just one) leaves it vulnerable to damage. Since the length you have to survive for is dependent on how often the boss makes the suicidal attacks, the fight is partially a Luck-Based Mission.
Set Pattern: The boss has a fixed cycle of attacks, and at some point renders itself vulnerable.
Analytical/Responsive: The boss will make itself vulnerable if given a certain situation. If the boss was run by a smart analytical AI, it would only use the vulnerable attack as a last resort, when no safe attack is possible (possibly not even then, if the boss doesn't care about letting the player live). But usually Artificial Stupidity is applied deliberately to prevent the game from becoming Unwinnable, especially if the boss always has safe attack options available. Alternatively, there may be a Tactical Rock-Paper-Scissors system where the boss defending itself from one kind of attack leaves it vulnerable to another, forcing the player to work out the right Combos of attacks to defeat it.

When it comes to video games, Deadly Dodging is nearly a sub-trope, as is Tennis Boss. Sister trope to Attack Its Weak Point. See also Boss Arena Idiocy.

Examples of Tactical Suicide Bosses include:

Portal justifies/lampshades this trope. The boss fight starts once you destroy GLaDOS's morality core, which prevented her from trying to kill you with neurotoxin. GLaDOS notes that a side effect of the destruction is that she can't turn her self-defense rocket turret off anymore. The turret, of course, is necessary to beat her, by redirecting the rockets through portals.
Portal 2: When you confront GLaDOS again, she's taken care to keep you from using the same tactics to beat her as in the first game. However, unbeknownst to her, you've already sabotaged her defenses.
- Mostly subverted during the true final boss battle, the boss informs you that it has analyzed the first game's climax and set up an arena with no portal surfaces and shields for itself... but left a conversion gel tube nearby to make some portal surfaces. After each time you hurt the boss, though, it reorients its shields to protect it from whatever tactic you had to use to hurt it last time. However, after taking some damage it informs you that it also cannot shut off the bombs now that it's recognized it as a liability.
Super Mario Bros.:
- Super Mario Bros: Mostly Boss Arena Idiocy, placing an axe at the end of his bridge even after seven fakes fell to their doom. But for this trope, his habit of making big slow jumps in the air can be incredibly helpful to the player, especially in later worlds.
- Super Mario Bros. 2: Wart opens his mouth so you can feed him veggies that kill him. To be fair, it is basically his only move. That doesn't excuse putting his own personal Kryptonite-dispenser right in front of him, though.
  - Which is actually the Dream-Making machine-thingamajig, which is self-aware and chooses to help Mario.
- Super Mario Bros 3: Bowser will keep ground pounding at Mario, even if it means his death.
- Super Mario World: If Bowser stopped throwing Mechakoopas after Mario, there'd be no way to defeat him.
- In New Super Mario Bros Wii