I understand how Mewtwo is unlocked, but I am curious if anyone had a method to reach 20 hours of versus play time or 700 total matches quickly.
You need either 20 hours of human time in VS mode, or a determined number of matches. (700 if I remember well) in order to challenge Mewtwo.
The first option is obviously easier, because of this reason : the human time piles up.
this means : more players = less time in VS for Mewtwo to appear !
CPU time doesn't count, so you'll need to plug in controllers, if you're alone.
With 2 players on screen, you need only 10 hours to get to the point of challenging Mewtwo.
3 players : 6 hours and 40 minutes
4 players : only 5 hours !
My recommended setup is : 4 players, no items, 1 life, listen to the music while AFK for 5 hours, on a stable stage, then everyone go SD except for your favourite character & controller who's going to challenge him.