To do this glitch, first pick Pikachu and Samus in Vs. Mode. I recommend putting Samus as the 2nd Player. Then, pick any stage you want. When the match starts, try to go up to Samus and use Pikachu's Neutral Special Move. This is the one where Pikachu send a ball of electricity to the opponent.

Now, pause and pause again while Samus is taking damage from Pikachu's move. Pause it when you see Samus change color. If you time it right, you'll see Samus Aran without her suit or anything else on. If you didn't, try to do it again.