Here's a solution I learned from the various solutions above me.
Make sure to have a second controller for this. Have your second controller be the infected.
Start Versus mode on the Crash Course level on Depot Finale and make it to the generator. Grab a weapon (preferably an assault rifle since it's more accurate) and another pistol so you have 2.
As the human player, survive until the generator blows out. Make sure to guide your Tank 2nd controller toward the survivors so they can kill it.
When the generator dies, run over and restart it and immediately begin jumping up the front of the generator until your right under the light. You'll be high enough up so that it will take the horde awhile to reach you.
Now here's the crucial part: The Tank (your second controller) will need to be near a survivor or else it will turn into an AI Tank. Have your Tank placed behind the generator so that the bar won't go down. As long as you don't get off the generator the bar won't go down.
Your AI partners will end up being inside of the truck so if you wish to kill them quickly with your Tank by attacking the side of the escape vehicle for the Wipefest achievement then go ahead but the survivors shouldn't be a burden since they'll be sitting in the truck.
Now use your pistols and just keep hammering away at the zombies as they come. They will attack one and a while if your not fast enough but they only do 1-2 damage.
I made the mistake of jumping off the generator to grab more assault rifle ammo when I didn't need it and I died from both the horde and the AI Tank.
In about 5 minutes I managed to net about 200-300 kills using the pistols so it won't take long if your about 10k-15k left.