Skywatch. Over by the crashed Hive ship. If you stand back towards the cliff where the warsat falls, and look uphill, you'll see a dark hole in the face of the hill. Kill the three acolytes by the container at the top of the hill, and they will begin to spawn. 3 acolytes, headshot them all before they leave the cave. Reload quick, cause 6 acolytes and 4 thralls spawn in <10 seconds. rinse and repeat. When you run out of ammo, run up to the cave before another group spawns and glory in the ammo and loot. sprint back to the rock, and continue. Its funnest if you can get a few guys to join you. Its like a day at the range, or going fishing. Good, manly fun.