The Dishwasher/YMMV

Game Breaker: The Violence Hammer in Vampire Smile. The first hit comes out slowly, but subsequent hits are faster and all do obscene damage. Most importantly, it's almost impossible for enemies to block. Since your opponents' one advantage over you is their ability to guard, this is kind of a big deal.
Goddamned Bats: Zombies. The games' way of making a boss a little bit harder is to throw an endless stream of the shuffling little bastards to mess up your maneuvering. Oh, and unlike most enemies, if a zombie is close enough to attack, he will always do so as soon as possible, meaning that they're going to be spending an awful lot of time knocking you out of vital combos if you neglect your positioning.