-Difficulty Setting (Scavenger)
-Zombies run (never run)
-Enemy Aggression (Feral)
-Daylight Length (Doesn't matter)
-Enemy Memory (90)
-Enemy Spawning (Very High)
Start up a game with these settings on. This works best if it is in a game that you have good equipment in so that you can kill zombies quickly. Get 72 small stones and make 9 fires. Next get a ton of wood, preferably at least 250 pieces for each campfire. Go to an open area and put the campfires down in a three by three square. Put the wood into each campfire and start them up. Usually a pair of screamer zombies will come in first. These zombies look like the little girl from the Ring. DO NOT KILL THESE! Let them live for about 30 second because they will call in more zombies. After a couple of minutes zombies should start flooding in from every direction. Be careful because its really easy for zombies to overwhelm you. This achievement will still take damn near forever but this method is much better than just running around trying to find zombies.