i think you have to defeat the vs. mode against a cpu. 700 times i saw it on a diffrent cheat site. good luck.

u have to play vs mode for 20 hrs

Ok, here's what ya' do.Before ya' go to bed (or school,bed works better)go to VS. mode, then go to where it usually says "2 Minute KO-fest" and click.Go to "item switch" and click. Go to the top to item flow, and put it at "none".Then press "B".(only once)Then go up to "rules", and put it on "stock:3". Press "B" once more, and plug in 2 controllers, and have 2 (you're best players) humans.(NO COMPUTERS) Press start. Go to the "pokemon stadium" stage and put both people on the pokeball, then unplug both controlers and set aside.(Do not turn off) GO TO SLEEP FOR PETE SAKE! In the morning, plug in both controlers. Have one run off the edge three times.Mewtwo will fight you.Beat him, and you can play as Mewtwo!