Can be done on solo private.
In the middle of the map, there are 2 glowing wormholes that you must kick or leash enemies into. Its important to only kill one enemy at a time in the portal, as it has to "cycle". Once the enemies corpse has exploded out in a fountain of blood, its ready for another. Make sure to kick them into the middle to guarantee the kill. If you get "Graffiti" or just a +10 kill - restart