Mewtwo with his feats from the anime and the series, and learned every technique (c'mon, he can't seriously just have 4 techniques, that's just the game mechanics) he can learn in-game from Generation 1 to the current Generation (of course no Gamesharking). I'm talking about only the moves that he can learn.
Mewtwo is virtually the most powerful Psychic Pokemon. Mewtwo exhibited a lot more skills than Mew or any other Psychic Pokemon ever had. He rebuilt New Island, can mind control, can teleport various Pokemons and even large pieces of land altogether, can erase memories, create a "planet-destroying" storm with a wave of his hand, shield himself from explosions, etc.
Which means this Mewtwo can use plenty of elemental attacks. Thunder, Earthquake, Hyperbeam (anime version), etc.
Moreover, Mewtwo has the capacity of setting up defensive wards such as Reflect, Guard Swap, and Safeguard, not to mention he can recover his HP when the battle takes a turn for the worse.
This Mewtwo would still be the bloodlusted one, and not the peaceful natured one at the end of the movies, though still having the feats of his peaceful natured self.
Psylocke you already know... oh and her martial arts fighting skills will be absolutely useless against Mewtwo.... and NO she won't have any Pokeballs of any sort. XD