9. Senator Armstrong - Metal Gear Rising: Revengeance
Senator Armstrong. A great boss, but also a damn annoying one. He is far from your conventional enemy, as is apt considering he's the final boss of the game, and is extremely difficult to deal damage to due to the nanomachines inside his body. Like many bosses on this list, the key is to always be on the move, because if you try to engage Armstrong for too long at once, he will eventually make mincemeat out of you. Get a few hits in, then run, and repeat until he's stunned and you can safely deal a good amount of damage. There's also a pesky QTE sequence to deal with, and one thing's for sure, in a game that is otherwise pretty easy to breeze through in a few short hours, this is one boss that'll keep you up until the early hours trying to beat him.