This is what i did,after the crash course dlc came out. You can also get the littlest genocide achievement on the way too. Using this method i racked up almost 2000 kills in an hour, and is i think way faster than the boomer trick in No Mercy. Just keep in mind you can only do this with the Crash Course dlc.
1. first I signed in 2 controllers in the main menu.(my account and then you can sign in just a guest.)
2. go into split screen versus, and choose crash course truck depot FINALE. Have your account as a survivor and the guest account as infected.
3. First, kill all other survivors.(Note: you can do this either in the beginning of the level or wait to kill them when you reach the generator. personally i just killed them in the beginning, just to get more kills through the level and also to keep all 4 health packs by the generator at the end of the level, otherwise they will take the packs if you choose to kill them by the generator.) anyways proceed through the level as you normally would until you get to the generator. as for the other controller, DO NOT spawn as an infected. This makes things alot easier to where you can just rack up kills without worrying about specials. just let the contoller sit there beside you.
4. Once you get to the generator and stocked up on health and ammo by the truck, only restart the generator ONCE ONLY, after that camp out at he other generator that is by the warehouse. (see video if youre unsure where this is) Now once you start the generator, 2 tanks will spawn, but do not worry, because the other controller is what is controlling the tanks. so when the spawn, grab the controller and take the tank over to where you are so that you can kill it without it attacking. you HAVE to do this rather quickly since you will notice that the controller will lose control of the tank if it does not attack within a certain time limit, but its plenty of time once you get the hang of it. do this for both tanks. after that the game will say to restart the generator. DO NOT do it. just camp at your corner and kill the horde that come to you. This is where you really rack up on kills. Each horde will consist of about 25-30 zombies and will come about every ten seconds, which gives you time to restock on ammo and health by the truck in front of you if you need. just keep doing this until you nab the achievement.
i put up a link to a video to show what im talking about camping on top of the other generator. Notice how the zombies don't attack from your left side, They come around and try to climb the right side only. This funneling of zombies will make it easier to not get hit. Just shoot them and while you reload,melee to keep them off the generator. Once you get the hang of it, you wont need health for awhile, but if you do its in front of you by the truck.
Saved me alot of time. hope this helps. credit for the video goes to cbkste.