To get this quickly, get together as many people as you can for a boosting session, but you can do this in player matches with as few as two people.
Load up a match on the map Boxcars. Have the shooter go up top in the area with the wooden scaffolding. There is a blown out room at the corner of the scaffolding with a Druganov sniper rifle that respawns infinitely. Have the shooter stand out on the corner of the wooden scaffolding and everyone else run out into the open area below and around the scaffolding to be shot. Set the match length to whatever you want, but 5 minute matches will rotate everyone quickly, and 200 kills is not that many, so you should have it within a few matches if you can get enough people. If you play on Deathmatch mode, the shooter will be able to see everyone on his mini-map when they come out into the open area.
Win or lose, you will also get credit toward the achievement for playing 50 matches online.