Finding one of the clues in Mombasa Streets counts as completing a level, so if you just play from the beginning of the game (preferably on Easy) and until you find the first clue (the helmet), you will get this with almost no fighting, making it easy to avoid using anything else than your pistol. Just make sure to not throw a plasma or spike grenade either, as they count as Covenant weapons.
Finding the helmet also nets you the first story-related achievement: