I've recently bought Street Fighter Anniversary. And that game is very tough just trying to get to Akuma is darn near impossible but when I finally manage to get to him he beats me every single time. It usually takes me hours and hours just to finally beat him and the game.
As tough as he is Akuma or Super-Akuma are also my 2 favourite characters in the games and I win all the time with Akuma he's the cheapest character to use. Even when I am fighting as him in the "X-Men: Children of the Atom" I am pretty much impossible to beat.
Magneto of "X-Men: Children of the Atom" another tought customer to face and another one with a whole lot of moves that make him very tough to beat and his Shockwave can knock you out if he does it twice in a row it does a whole lot of damage.
Magician: House of the Dead: Very tricky in beating. You always have to be very fast to beat him in the end of the House Of The Dead Game.
Edited: To let people know they can put in as many boss characters they choose. I'm sure there's alot of tought boss characters out there. Put down as many hard bosses you want.


