Mysterious Figure

The biggest dick in all gaming history.

He's an optional boss in kingdom hearts, birth by sleep. He has 2 lightning energy dagger type weapons and he can cast pretty much all the strongest spells such as meteor, megaflare, death, and all of his melee combos bring you to 1hp almost, he also turns invisible at half health and you can no longer see or lock on to him or see where he's coming from, he can make multiple clones of himself that can all do the same attacks and he has an attack that erases all of your special attacks.

Also if you start doing a moderate amount of damage to him he will either put up a temporary shield that makes any damage heal him or he will stop time, reverse it, teleport behind you, begin an attack, and then unfreeze time again.

One of two secret bosses from the PSP game Kingdom Hearts: Birth by Sleep.

The Mysterious Figure fights with two glowing blades, similar in design and function to those of Xemnas. Alongside his regular attacks, the Mysterious Figure can also freeze and rewind time, make clones of himself, and cast several high level spells, such as Mega Flare and Meteor. The boss's most lethal attack is a jumping "lasso" move, which not only does significant damage, but almost always leaves the player open to a second, fatal hit immediately afterward. These attacks, combined with the Mysterious Figure's high speed and resilience, allow him to officially outclass Sephiroth as the hardest boss in the Kingdom Hearts series, and possibly the hardest in all of gaming history.

Those who can defeat the Mysterious Figure on the highest difficulty with all players will earn the right to call themselves a hardcore gamer.

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.