This is a great way to farm kills if you don't have the DLC. Load up Chapter 7: Tourist Trap. After you Step Jump over the wall, clear a path along the right side with your partner to the Mounted Gun. Have your Partner watch behind you to the left and to your sides while you blast away the other enemies as they come in through the spawn points. Once you clear out all the enemies, two Shotgun Heavies will come in, one at a time. After finishing them off, you are finally ready to start.
First, face the barrel end of the Mounted Gun to flip it around to face the fence. Focus on the right side while your partner makes sure no enemies make it down there from the left side with as little killing as possible (With an AI partner, just have them hold position next to you). Clear out any enemies running on or at the bottom of the stairs. Once this is done, point the Mounted Gun at the top of the stairs and kill any enemies you see. Enemies will be spawning from the left side in groups of about 5 and if you have all the aggro, they will be running into your view to try to get a shot off on you. I did this a few times and got over 100 kills each time before getting bored or having to restart from checkpoint. As long as you stay on your side of the fence and don't try to run up the stairs, the Gatling Gun Heavy won't spawn and the other enemies will keep coming.