Partway through Mission 2: Second Chance, you will learn how to use the armor ability. Upon reaching a roof, you will notice a donut to your left and 4 enemies on the ground. If done fast enough, you can knock over the donut right away and get a kill, but if you want to take the safe route, makr them with your visor and wait for their conversation to end. Once soldier will begin to walk back to his position right under the donut an next to a car. If you melee and shoot the donut, it will fall explode the car and kill the enemy. Wait a few seconds and you'll get the achievement.