Killing 37,000 zombies an hour
First, get the Sling Rot combo vehicle blueprint located on the side staircase of the church, which is located on the bottom-right end of Old Town. You will also need all five bottom row skills in the Brawling section. Create the Sling Rot combo vehicle using the snowplow truck and hearse found directly in front of the church. They can also be found on the highway behind the church. Drive the Sling Rot in a perfect square around Old Town for the first loop, killing as many zombies as possible. Drive it on the inside streets of the square you just drove for the second loop. Drive it up to the bridge on the highway (behind the church) on the left-side for the third loop, then turn around and drive it back to the church. Your vehicle should have very little health remaining once you reach the church after driving those loops. The vehicle and zombies will now have respawned. All three loops take approximately three minutes and get close to 2,000 zombie kills. Keep repeating this to kill approximately 37,000 zombies an hour. Additionally, make sure to keep reaching the 75 kill combo to get the special flame ability for the vehicle to kill a lot more zombies. Use the special flame ability on large hordes of zombies to easily kill them. Use this method to get the “Z-Genocider 4: The Return” and “Left 100,004 Dead 2” achievements and complete the trial for getting 250,000 zombie kills.