Start "The Ark" mission and have a guest or a friend to help. Proceed until you reach where the crashed Pelican is found (after the first terminal). Kill the blue Brute and take the rocket launcher under the crashed Pelican, then continue to where the choppers are located. As you are moving, a Prowler will drive by. Ignore it because it will soon stop and will abandon the Prowler, leading to failure of the achievement. Another Prowler will follow. Have your guest or friend shoot it twice with a Brute Shot, then shoot at it with your rockets. The achievement is yours. Note: If you are by yourself, do the same thing but instead of using a Brute Shot throw a grenade in front of it. Then, shoot it with the rockets.