This achievement is story related. It is awarded for completing Half-Life 2 by destroying the Citadel's reactor core before Dr. Breen completes his teleportation. Make your way up to the top of the Citadel by passing a series of energy ball fields, combine soldiers and moving platforms. Once you reach the top, you are confronted by 2 Combine gunships. You will notice that 2 energy ball fields appear in front of you. Use your supercharged gravity gun to destroy the reactor core by shooting multiple energy balls at it from the side.