Luigi wins by doing absolutely nothing.

If you play agains the lv 9 CPU on those maps without moving, they ALWAYS do that moveset leaving to their own demise. At least some/most of them. Lv 9 CPU in melee was known for being quite weird.

EDIT: This is more evident on lv 9 Fox. Where since he doesn't even touch Luigi, gets trapped on a loop, since both starting positions set the moves in action. I remember doing it on my own game, and if you have items on, it's enough to break the loop. Some others were just guesses. Oh, and it's not only with Luigi, works with anyone if I remember correctly.

This also works with average-sized characters with lv4 opponent.