Find the Berserk power up on a map in multiplayer and wait for it to respawn and then rinse and repeat. It takes super powerups around 90 seconds to respawn. You could get this more easily with another controller as you don't have to be worried about getting attacked. Setting to game to 1 hour will help.
Note: You can also get this by just standing where the powerup respawns and then doing other things like watching TV until you get it.
Here is a link to a thread on the locations of powerups: POWER UP LOCATIONS.