After storming the boat (during Act 2), you will reach the door to gain access and will be attacked by walkers from behind. DO NOT USE hand to hand combat. Instead USE your bow and arrow, keep distance and kill all walkers using headshots to unlock this. The following missable achievement can be seen in the video below at 10:54
If a walker grabs you, let them kill you, and retry the checkpoint. Do not use the button prompts when they are close.