During the first level you will meet enemies equipped with rocket launchers. The game will prompt you to press LT (left trigger) at the moment of impact to reflect a projectile. Get the timing right, the projectile will return (in slow motion) and blow up the person who fired it. Pop goes the achievement.
(It's also worth learning to dodge with the right analogue stick to get a clear line of sight and make room - don't get too close as it will give you less time to reflect the shot).