Unlocked this recently by joining a
Private: King of the Hill match with 2 other friends. (This also makes it easy for them to get the achievement after you do, just take turns defeating each other, using Harley)
How To Unlock:As Harley, I got my friend down to critical health, and then used her character trait. Make sure you
Press and Hold Down until both her Hyenas start running.
Do this as the killing blow, and the achievement will unlock. Happy Hunting!