Doing Work Achievement
Kill 1000 enemies while in Guerrilla mode
You can obtain this on any difficulty and level. This will come over time while you play Guerrilla mode. There are 50 waves for each level. So depending on how many waves you complete. Playing with a full 4 player lobby might limit the amount of kills you would get, so doing this with 2 players would have a better out come. That way there is an even amount of kills shared.