Stadium Cheats

Black Pokemon cheat

At game title screen quickly press up, up, up, right, up, left, down.
After you should hear a pika, pika noise.
Now all of your Pokemon are black or darker then normal

Extra Damage
After performing a fighting move, hold A when the move description appears.

Get Mewtwo

To get Mewtwo, beat all of the trainers in tournament mode (it's in the upper-left hand corner of the Map).
Once you beat Giovanni (the 8th one), the upper-left hand corner will be revealed.
Go up there and beat the Elite 4 AND Gary, all in one run.
Then fight Mewtwo, beat it, and you can then use Mewtwo.