Dr Fetus was pretty hard, in Super Meat boy. Took me around 2 hours to beat him, and seeing as an average life was 20 seconds or so, thats around 300 tries. Though I don't think Fetus himself ever killed me- the level did that.
After that would be the final boss of FF13-2. The previous 3 bosses (This being a 4 part boss) had been easy as cake, and then this guy shows up and it takes me 30 minutes to kill him. He has around 3 1HKO moves, which can only be avoided by putting everyone in sentinel, and he is only able to be damaged when his buddies are dead. When you kill them, you get to fight him for 2 minutes (less, since he starts by spamming daze) and then he respawns them. And god forbid he should use his signature move when both his buddies are alive- if you're not overleveled, that can 1HKO you through sentinel.
After that would be the final boss of FF13-2. The previous 3 bosses (This being a 4 part boss) had been easy as cake, and then this guy shows up and it takes me 30 minutes to kill him. He has around 3 1HKO moves, which can only be avoided by putting everyone in sentinel, and he is only able to be damaged when his buddies are dead. When you kill them, you get to fight him for 2 minutes (less, since he starts by spamming daze) and then he respawns them. And god forbid he should use his signature move when both his buddies are alive- if you're not overleveled, that can 1HKO you through sentinel.