When Covenant dropship is getting ready to unload a group of Covenants, throw a grenade into the door as it opens. The blast will kill several of the enemies since they are all in one area. There are places (especially in the first level) where a groups of Covenants are, waiting for you to barge in and start shooting. If you can, sneak to the aliens and throw a grenade into their midst. It is best when a plasma grenade sticks to a Grunt, because he will run behind the rest of his companions. One simple grenade can take out five Covenants.