The bold ever fight on, their lives echoed in song
Damage sponge fights. Now, I don't mean "has high HP but is reasonably manageable if you learn the game/fight mechanics", and I don't mean "there's a gimmick that cuts big chunks out of the fight so it's mostly irrelevant", or even "scripted hopeless boss fight you're not actually supposed to win anyway" (although I'm not overly fond of those, either). I mean enemies that have fifteen quintillion HP for no valid reason other than to drag a fight out forever. It's a waste of time, it's not fun, and it's just annoying.