biehlza

Best answer as chosen by Asker / Ratings
I believe there are 2 ways to unlock Mewtwo.
You can either play Vs. Mode for 20 hours (combined time)
Or you can have 700 Vs. Mode matches, then he'll fight you, and if you win, you unlock him.

simpsonsfreak

Best answer as chosen by Asker / Ratings
Easy trick if you have at least two controllers: Plug in as many controllers as you have, set up a melee match with five lives (stock)for each character (make sure one of them is your best character) and select a stage with no movement whatsoever like hyrule temple (you might want to take off all items too). Each character should be human, not computer controlled. Start the match, and leave the system on all night. The next day, kill off the other characters using your best one and when the match ends, you should fight mewtwo to unlock him. If not, repeat.

maniacman1234567

You can play either 20 hours of combined play time or by playing 700 verses matches. Then he will challenge you.