To get Mewtwo, you must play vs. mode for over 20 hours. This is easy to do. Select a 1-minute time mode melee game with the stage as Hyrule Temple. (By the way, make sure that both characters are humans or else this won't work.) Don't attack each other and wait until you go into sudden death. in sudden death, both players go down into the tunnel underneath the temple ruins. Wait until the bombs start falling to make sure you're safe, and if you are, pause and leave the game system on for about one day, but make sure something has the joystick on your controller pushed to the left or right. (This will keep the image moving so the TV doesn't get screen burn) After a day, finish the match and if the game has been left on for long enough Mewtwo will challenge you.