200 opponents means either the Slayer or Demon players - both count toward your total. Using the following boosting method you will get about 25-30 kills every 10 minutes. You can do this in a private lobby.
The person going for the achievement must be the Slayer. The two boosting partners should be Revenants, because they have the least health of all demons which leads to faster kills.
Slayer kills Demon player #1, and in the 20-25 seconds that it will take for that player to respawn and return, weaken Demon player #2 to very low health. As soon as the Demon player #1 returns, kill Demon Player #2. Rinse and repeat.
During this process the Slayer will be constantly being attacked by demons - just kill them to refill on health and ammo.
My boosting group was taking 10 minute turns, but rounds seem to extend beyond 10 minutes as long as both Demons aren't killed simultaneously, which automatically ends the round. I can confirm that the achievement will pop during the match.
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