Guyra125,552 29 Apr 2017 25 Feb 2021 25 Feb 2021
Early in Mission 2 there is a box on top of a drop down towards several zombies and survivors. Break it open to reveal a gun wielding orangutan that'll help you out. This will unlock the achievement.
This isn't the only place to unlock this achievement, but it might be the easiest place to do it.