Nightmare in the original DMC, 3rd time you fight him on DMD mode. Literally the most retarded boss ever designed in any game purely because his difficulty is based on how much you damage his cores the previous two times you fight him, He's fucking hard even when they take full damage, and I am pretty sure he's close to impossible when one of his Cores is red. I had to restart my game purely because I didn't understand how the boss worked.
I'm normally quite calm when it comes to frustrating parts of a game, but I'd be lying if I said I didn't lose my temper on multiple occasions trying to do this