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A look through the Boss Battle index suggests Cat-and-Mouse Boss.
Edited by HeraldAlberich
^ That's only if the boss is chasing you. What method exactly is used to make it vulnerable determines what trope it is. Puzzle Boss and Tactical Suicide Boss are both likely.
Edited by pikachu17 17 pikachus all in a row.
For both Puzzle Boss and Tactical Suicide Boss, once you've either solved the puzzle or tricked the boss into hurting itself, the boss is automatically damaged by it. What I'm thinking of is not the boss being damaged by you doing the thing, but the boss being made able to be damaged - You might have to hurry to get in as many attacks as possible before it becomes invincible again, and it might still be attacking you even while it's vulnerable.
thanks, that seems to be it
A boss where if you damage it enough, it becomes invincible, and you have to do something (e.g. kill some other enemies, do some platforming, survive an onslaught of powerful attacks, etc.) to return it to its vulnerable state. Then, you damage it some more until it once again becomes invincible, and the cycle starts again. Cowardly Boss would be a subtrope.