with an other crystal beast deck!
You have to beat them.
you beat it
nothing, the credits will roll as if you beat the game, but you get nothing
beat part 1,2,and3
meet him and beat him
you can go back and beat him again the next day!
you can beat them has much as you want
get rare Pokemon and beat him with them!
You battle
myyy but is big
Crystal does not have Mewtwo. Instead it has Ho-oh after you beat the Pokemon league.
U pretty well just beat the game.