This is not nearly as hard as the retail games. I did it on my single player playthrough when I went for Savior. In the main security room there are always a lot of zombies on the ground floor. Just spend a little time killing them there and it'll come in no time. I used a combination of shooting all the explosives lying around and would occasionally grab the little golf cart by the safe house and drive over them. You can also combine a shock prod and floor buffer to create the Zap N' Prod, with which you can get you around 100 kills before it breaks. Don't forget to do the case files in between killing. If you don't, your game will end and you'll have to start over.