You will need the combo card for the Boomstick to get this. You can get the combo card from getting gold on the first challenge on the roof of the Atlantica Casino in Sandbox mode. See "Challenge Domination" if your having trouble.
You will then need to create a Boomstick. It is easily created in the Palisades Mall, from the entrance to the Palisades from the Atlantica Casino there's a pitchfork in the dirt in between the two escalators. The gun shop upstairs, High Noon Shooting Range, has a shotgun in it. Combine them both in the combo room downstairs to create the Boomstick and use the heavy attack by holding while next to a zombie. This will net you 2,500 PP.