During Mission 1: Fog of War
, your squad will break through a blue door, and one of them will be hit be a bullet from a machine gun. Work your way to the left, and around the area taking out any enemies along the way until you reach the staircase leading to the machine gunner. Throw a grenade to take them out and score this achievement.
Possible on any difficulty, but recruit is probably recommended to make it as easy as possible.