This requires enemies that are friendly to each other to kill each other. Prodean units killing Chaos units will not count. You can get enemies to kill each other by lining them up and having them attack each other, especially easy with stronger enemies like the Slayer.
However, if you'd like to speed this up, I have created a custom map for it. In the Community workshop, searching "friendly" or "achievement" or my username will bring up the map (Actual Friendly Fire Map). Just walk forward and wait for the enemies to kill each other, and go to the corner with the rune to exit.
Thanks to Theportal2010 for the help!