Easily gotten at the very start of chapter 2: "Direct Intervention". If you've already played through the game, you can simply start a new game here.
Either way, once the large door opens, move straight ahead, and two Elite Soldiers will come from opposite sides to attack. At least one of them will fire an energy ball in the first few volleys. As soon as they do, use the gravity gun to grab the energy ball. Then simply fire it back against the Elite Soldier that fired it.
Even with a couple of tries, unlocking this achievement should take only about a minute with this method.