Mysterious Figure

Quite possibly the hardest, and most certainly the cheapest, boss in all of videogame history. Making his first appearance in Kingdom Hearts Birth By Sleep, he was a mysterious hooded figure who wielded two ethereal blades and had the ability to two shot the player on the lower difficulty settings, and one shot the player on the highest. His sheer damage output, coupled with his ability to start an attack mid combo due to teleportation, and his ability to literally be anywhere on the battlefield at any point in time, often to the point of being in two places at once, made his fight much harder than was necessary. Couple this with the ability to create four copies of himself, cast the highest level spells in the game, and one-shot the player at any point and time and the battle became more a matter of "when" you would suffer defeat, as opposed to "if". His subsequent appearance in Kingdom Hearts 3D was a mocking nod to his previous appearance, although not nearly as difficult, if defeated, he would rewind the battle all the way back to the opening camera with half of his health restored and force you to fight him again. Due to his cheapness, he is definitely the cheapest boss in videogaming history, and ironically, quite possibly the hardest, said irony coming from the fact that the game itself is quite easy, even on the hardest setting.