Most annoying boss, Vaati from Minish Cap, you have to get to him facing about 4 or so dark nuts before 3 bells ring then if you die you have to start from the very beginning even if you did reach him. Oh and the amount of transformations he did just made it worse.
Bass Megaman Battle Network series.
Any boss with a takes up 3/4 of your health, is unblockable and can only be avoided if you move at exactly a 10th of a second before it hits you and is only used by the boss when it is almost dead.
Bass Megaman Battle Network series.
Any boss with a takes up 3/4 of your health, is unblockable and can only be avoided if you move at exactly a 10th of a second before it hits you and is only used by the boss when it is almost dead.