The tentacle boss-woman-thing from Prototype. The entire game was a cakewalk even on the high difficulties apart from that one mission. She had 4 separate health bars, and the first 3 were for her 3 tentacles, and the entire time you are trying to whittle them down she spams an attack in which she just explodes. The only way to dodge this attack is to run away, meaning it takes even longer to bring the health down.
When you finally take out those 3 tentacles, you get a (very brief) window of opportunity to hurt her real health bar, and then all 3 tentacles regenerate themselves and you get to repeat the whole thing about 50 more times before she finally dies.
I will never play that level again. I'm sure that one boss took hours, and it wasn't fun at any point, just tedious.