1. During the mission War Pigs, pick up a shotgun as soon as possible.
2. After that, at a certain point you are ordered to walk up the stairs (right after you have to use your night vision goggles). The stairs make a 180 degrees turn and consist of 2 sets of stairs (like this:
https://www.pinterest.com/pin/62065301084246685/). On the part in the middle, there will be an enemy waiting. Kill him immediately, before he has the change to shoot. After you kill him, the achievement will be granted.