This method will allow you to kill approximately 1,200+ zombies every 5-8 minutes. Keep your zombie score multiplier running to get bonus PP and scrap for reaching milestones. Go to the church in south-eastern West Ridge. This area contains huge hordes of zombies. Near the steps is a military platform with an Exo Suit. Grab it, then get the Vacuum upgrade for the Exo Suit to the left of the stairs. Kill as many zombies as possible while circling the church. You can easily get 1,200+ zombie kills before the power runs out. Once the power runs out on the Exo Suit, enter the nearby van and drive it to the bridge connecting West Ridge to Old Town. This will activate a checkpoint. Once a checkpoint is activated, everything in the area will be reset. Return to the church in West Ridge and collect the Exo Suit and Vacuum upgrade again. Repeat this process as many times as desired. The level cap is 100.