Up Throwing the opponents is strangely very effective here, and can kill them in one hit. Rest also has a strange tendency to work.
Using Jigglypuff's Pound works if you hit one at a time and dodge the others. Use Pound get through the Jigglypuffs and get items to easily hit them off
If Jigglypuff uses Sing on this stage while the screen in the background of Pokemon Stadium is focused around it, all three of the other Jigglypuffs on the ground will fall asleep, and for much longer than they usually would. This easter egg is known as sleepy time. The effect is not unlike Peach Blossom, Peach's Final Smash in Brawl.