I never was all that great with Mewtwo. He's a very evasive character, and pretty smooth in motion, but not quick and sharp enough for my liking. Any character can be used and taken advantage of, and I'm very sure you could master Mewtwo in this way also. I do know that Shadow Ball is a beast when fully powered up though.
Teleport can get you out of danger when needed too.
Not a bad character at all.
As for unlocking, you need to have played 700 full Vs. matches, or 20 hours of Vs. mode. If you really want Mewtwo (and the other characters) the fastest you can, just play X hundred Super Sudden Death, 1 life matches. 700 one-hit KOs come a lot more quickly than 700 done any other way.
Note, the 20 hours would be split between human players. You could just have 2 people playing each other for 10 hours.
Hehe... before I forget, here's something I remember someone telling me they did...
Before you go to bed for the night, just open up a Vs. game, human vs. human, unlimited time match, on a stage that has no hazards. Just put both characters on safe ground, go to bed, and there's at least 16 hours towards your 20. XD