With amerc auto rifle, go to the Tunemakers music store at the Royal Flush Plaza. Make the Blitzkrieg weapon by first combining the wheelchair (found just in front of the store) and the car battery (found inside the maintenance room), then combine the electric chair with the merc rifle. Ride the Blitzkrieg out into the plaza and shoot zombies. When you're out of ammo, drive the Blitzkrieg through the opening to the Slot Ranch Casino. Ammo for the Blitzkrieg is now refilled and you can shoot more zombies. When you run out, just drive back to the Royal Flush Plaza and the ammo will be refilled. Repeat this as many times as desired for easy PP. Note: Do not run over too many zombies because the Blitzkrieg will break.