You have to be in a darkness zone of the nightfall strike. You also have to get kills using fusion rifles. In this weeks Nightfall that means once you enter the refinery area where the Hive and Fallen are battling it out.

You could farm the first section of enemies (the hallway before the 3 waves area) in this weeks nightfall solo, then just go to orbit and restart the nightfall until you get your 200 kills. If you happen to have a void fusion rifle, it would go relatively quick. Farm the enemies in the rocketyard area before going in for ammo drops. You do have to be killing level 28 enemies for them to count. Farming the public area before the strike 'starts' doesn't count toward the bounty.

Try using a fusion rifle and hope for weak opponents. And get some good PvP clan members to help.