There is a decent chance this will unlock through normal play as there as several levels which feature enemies killing each other. Prodean (blue guys) enemies killing normal Chaos enemies counts for this achievement.
However, if you'd like to speed this achievement up, I have created a custom map for it. In the Community workshop, searching "achievement" or "in-fight" or my username will bring up the map (In-Flight 100 Achievement). Just walk forward and wait for the enemies to kill each other, and go to the corner with the rune to exit. You'll have to repeat the map 3-4 times. Will only take a few minutes.
Thanks to ThePortal2010 for the help!