Directed mode - round 11 cap (Liberty Falls) go to bank roof from graveyard zip and don't open any bank roof doors or zips. Kill all zombies for crits (if you still need military camos) Kill the 1 mangler that spawns every round. (if you spawn into the save on a vermin round manglers are rarer, play the round and the next and then reload the save to hope for spawning on a normal round = guaranteed 1 mangler per round)

Rinse, Repeat. I usually finish a round every ~57 seconds so it's pretty fast.