First off, you'll need to create a custom stage for the glitch to work. In the Stage Builder, create a small 'room' that has 5 solid blocks around it on the left, right, top, and upper 2 corners. Seal the bottom off with a thin platform. Ideally, it should be such that if a character jumps into the room, they can't get out unless they duck through the floor. Create a flat ground level underneath it so that you can throw items into it.
Now, turn items on high with Pokeballs only. Start a match in your new level. When you get a Pokeball, throw it into the small room you made. Something will obviously be wrong when you get the more common Metagross, as he violently shakes up and down in the room since he's larger than it. However, in the rare case that you get a Groudon, he'll become literally stuck. He'll emergae and stay perfectly still. Actually, opponents can even touch him without taking damage, and he'll stay there for the rest of the match. However, one less pokeball will be able to be summoned up, so the max amount of frozen Groudons you can have is 3. This looks coolest if you go back and make 3 'rooms' in your stage.
Submitted by Artemis251.