At the end of Act 4-1 you receive directions to get into the elevator after holding down the outdoor area for a while. You can go into the elevator and camp, killing all of the robots that spawn. They come for quite a while but they do stop eventually. Just make sure you ignore the order to start the elevator by pushing the switch. After the achievement unlocks, you can reload your checkpoint to right before it tells you to get into the elevator saving yourself all the ammo you used.