Nothing counts as more than one kill. Not even the boss. My suggestion is go in solo.....everything scales down to there only being one person inside. Go into the first room and clear as much as you can but die before you get to the "No respawn zone" Once you hit that zone, if you die, you get sent back to orbit. If you stay in the first room and die you will not get sendt back. I was able to get about 20-30 kills per run before suicide and repeat. I got the quest after I had already cleared the Nightfall for the week....so I just did it this way....wasn't too terrible.