To get this achievement, you must destroy all four robots outside at Vandenberg -before- going inside the building. You'll need to start the level with the GEP gun, a lot of LAMs and/or some EMP/scramble grenades. Simply pick off the robots from the roof (or scale your way down to ground level) to get the achievement. A sniper rifle is helpful, as the guards on the towers will snipe you if they see you. Otherwise, be careful to stay out of sight.