In Killzone 3, in singleplayer, use the secondary fire function of the WASP launcher to kill a single Helghast. Chapter 5 “Icy Incursion” – Providence Bay, when the section starts, run to the other end, clear the area and go up the stairs in the building. Clear out all enemies and you’ll be asked to take the WASP, which is a mobile rocket launcher, to destroy the Gun Nests. Detach the WASP, press to zoom and let the gun autolock on any lone Helghast and fire away to your trophy.