We think this works with any enemy, but we've only tried it with wrigglers and goombas. Bring a spring to a place in the enemy's trajectory. Now, time a jump so that when you land on the spring, the enemy will be right in front of it. Although jumping on a spring bounces you and jumping on a badguy injures the badguy, neither will happen and you will in fact power down (or if you're little Mario... you'll die).
Submitted by: Connie