You will need a friend for this method and it makes it much easier in general.
Start a private escalation and kill every enemy except for the last one or two. On the map Ruin, which is what I used, there is a crate in the underground area which you can use to change your ability or get ammo. The last enemy will rarely come in if at all or will run away when you look at it.
I would equip the Chaos combuster and shoot at myself while standing in the corner to hurt myself. Be careful as getting trigger happy will just kill you. My friend would then heal me. Rinse and repeat, took us around 20 minutes or so.
Hopefully this helps anyone left that may still be playing this game.
You can also keep doing this to get the "Give Me The News Doc" achievement.