You can always try the Dead Air campaign. I don't remember which safe room it is, but one of them has a boomer inside. Now if you have a friend or a 2nd controller, have them go into the safe room with the boomer and close the door. Have him go up the ladder and all the way in the back right. The boomer will follow but he can't harm the other player. Now facing the safe room door on the outside, go left and crouch in the corner right next to the door. Every 15-20 seconds I had a group of about 20 run in towards the door. They rarely ran after me. The boomer will puke on the player spawning a group of zombies that will want to attack him. There will also be random groups of zombies that will run to the door. I don't remember if the ammo is inside the safe room or around the corner to your right. If it is in the safe room, only go and get it once you have killed the group of zombies. Quickly open the door, grab ammo, and close the door and go back in your corner. I believe it is a faster method then the No Mercy finale safe room, but it requires you to be active.