Load up the level "The Silent Cartographer" in Halo:CE, and set the game to easy difficulty. Immediately after jumping off the pelican in the beginning, head left. Keep heading this direction until you come close to the shoreline (to the left of the large hill). After this, you should be able to see a big cliff. Keep heading in that direction, and once you reach the cliff, turn right and you should see a large group of jackals. Simply throw a grenade at them, and most (if not all of them) should die. This will net you around 5-6 jackal kills with a proper grenade throw. After this, hit start, reload the last checkpoint, and you should spawn fairly close to that group of jackals again. Rinse and repeat and this achievement should come with a bit of grinding! I can also confirm that this counts towards achievements for campaign medals, although I'd advise trying to get that through natural progression instead.