Have one player go buy or pick up a shrinking potion. Enter any normal stage(no bosses), and let enemies attack you. Do not activate the shrink yet. Make sure the player holding the shrink is the last one alive, and when he/she is at about 30 health, activate the shrink and keep it on until that player dies. Then, exit the level, sell the shrinking potion, go to any stage, and the enemies should be permanently shrunken. If another shrinking potion is activated at any time during the glitch, the glitch will be deactivated.