Near the beginning of Mission 9: Night Shift, you will repel down a building and follow your teammate behind a car. If you look down the alley where he is looking, there will be 3 enemies standing there. Just line up 2 of them, and shoot. You will unlock the achievement, and also fail the objective, but it will reload the checkpoint for you after you do this.
Please see the video for a detailed visual of how to get this achievement: