On the level Skin, you just have two people in your team (Black and Jones) for the Ki/Ninlil boss fight. Dispose of the fireball throwing boss and just leave the one that drops a cultist off every 10 seconds.
Play as Black (might be easier to let Jones get downed), and just use ghost bullet to headshot the cultist when it starts to run at you. There's an infinite supply of cultists, so it will pop eventually (will also get the ghost bullet kills as well)
You have sufficient time for your ghost bullet to become active again before the next cultist arrives.