The moon. Take the mission 'The Dark Beyond', lvl 9, then play until you get a cutscene with a dead guardian (only takes about 2 minutes to reach). After that, doors open and a LOT of Hive pour out. You just stand in the spot you spawned in and shoot everything you see, EXCEPT the Hallowed Knights (killing them advances the mission and you don't want that). The hive will overrun you and kill you and you respawn in that same spot. You'll be shooting about 50 guys per minute, and that includes respawntime. And since it's an instanced event, no asshats can come and ruin your spawn like they do on the patrol missions