To get this achievement you need to finish the game with the bare minimum ending. This means not doing the quest to kill the punk boss, and not getting the secret weapon. You get the secret weapon by reaching area 15 with the bunker for the final quest in the game, and then choosing to continue on and get to area 20.
So to get this ending don't do the quest to beat the punk boss, and when you reach area 15 on day 42, choose not to continue on.