SurvivorG 20

To unlock this achievement you simply need to complete the very last mission in the original game (The two new DLC game types do not count) called The Wall. The mission has you defending one of the survivor in waves as they rig explosives to destroy the wall that is blocking your escape. When this mission pops up there is no rush to complete it. You can continue exploring the rest of the world until you are ready to leave. Once the mission is completed and the game ends you can also re-enter the game on the last save point before completing the The Wall mission.