MuppeT May Cry's solution is helpful and very detailed, but I just wanted to share a video of how I managed to unlock this in case it might help anyone.
I recommend finding an isolated, squishy (or at least low health) enemy with a good amount of air above their heads (so they fly up real high). I'd also avoid anyone with an escape/movement ability like Reaper, Moira, or Tracer, just to make sure they fly up and float back down in a relatively straight line. In my case, my victim was actually red team Ashe ;)
Hope this helps!