It IS possible in SSBM, but you have to do it in a place where the shells can't spread out. For example, go to Green Greens, and shape the blocks on one side like a U, with you in the middle. Then put the enemy under the "Human" setting, and move it into the U also. Then fill it with shells and use an attack that does not damage the surrounding blocks too much... the audio stops for a few seconds if you did it right, and you usually havve 100+ combo and 999% damage a piece.