This can be done anywhere except Versus mode. You must kill 53,595 zombies. When playing in Campaign mode, get a couple friends (Louis, Bill, Zoey, and Francis) to help you. The bots can sometimes kill the zombies and special Infected more than you can. In order to kill more zombies than the bots, find a sniper rifle, get to a high area, and snipe each zombie. You can also go lower, but turn off the flashlight or your snipe may be ruined by the attack of a zombie. The best levels to do this are any until the finale in "Blood Harvest"; "The Greenhouse" and "Crane" levels in "Dead Air"; "The Church" and "The Town" in "Death Toll"; and the first three levels in "No Mercy".