Arrow Cam is your friend!
That said the way I did this was standing around at the border of "return to battlefield" zones, firing over trees and buildings and using arrow cam to see where it lands.
- Fun Fact, After the Cam switches back your default firing screen hasnt moved.
So just fire wildly into the air making little adjustments and EVENTUALLY you will hit someone, then If youre lucky, EVENTUALLY that person you hit will already be damaged, and the arrow will kill him.
I hit 3 dudes in the leg from 120m over a 10hr session before I finally got it done. Most map areas spawn you closer than 100m from the enemy, so you might need to back up a bit ;D
Ps. Ballistas Don't Count - Goodluck!