In several areas throughout the game you'll have to block antlion holes with wrecked cars to stop them from spawning. In any of these areas, just use your gravity gun to smash the antlions with the wrecked cars. You can literally just move a car back and forth on top of a hole spawning them, though you might need to stun them now and then before throwing the car at them.
After killing 15 antlions this way, the achievement will pop!