The one thing I should mention is that this achievement doesn't have to be done on enemy characters, it can be done on friendly targets.
All I did was find a couple rakyat standing on a beach, equip my bow with the marksman sight, equip explosive arrows, line up the sight at the 80 yard mark and fire, if it went to far past them I backed up and tried again. Took 5 explosive arrows and I got it.