Start Half Life 2: Episode 1. Chapter 2: Direct Intervention.
When the level loads, go through the door ahead of you. Four or five Elite Soldiers will attack you. Stand back and don't shoot! One of the soldiers will shoot an energy ball at you. Grab the energy ball with your gravity gun (LT) and shoot it back (RT) at the same soldier.
The achievement will unlock if you hit the same Elite Soldier that shot the energy ball. If you miss or hit the wrong soldier, press start, select New Game and reload the level.
I have now uploaded a video to show just how quick this method is. Good luck!Please either comment below or send me a Personal Message if you have any comments or improvements.