The first mission on the moon where you find the dead Guardian. I always do the bounty during that mission, when you get to the final fight there are 4 majors, if you kill the Hive Acolytes and leave the Knight alive, then die and respawn, you kill the 3 Acolytes again, then the 3rd time you can kill all 4. Then leave back to orbit and you will have a checkpoint at that part of the mission for next time. Easy. Done in 5 minutes. EDIT: This method is also the quickest way to get the "2 kills with a Fusion Rifle" bounty.