Zombie Army 4: Dead War

Simply set up a horde match on any map with 1 player zombie spawns (The first one was the easiest by far) preferably with a group in private co op and simply play until wave 20 while gathering all the heavy guns dropped by the heavy zombies and putting them near the back of the map by the map exit for the later rounds 13+. once you hit wave 20 it should unlock even if you don't exit the map and finish the match. Thanks to my step bros (Shizblacka, Cully, Chrisaldin for the help)