Yeah, it is somthing hard to make. Its because oblivion doesn't register where an arrow hits, it just checks if it hit or not. Also, checking to see if it hits the head in particular is hard, becuase oblivion also doesn't know what constitues as "a head". Location damage kinda worked for me, but as well as making NPC's head a more damaging target, it made every creature in the game near invincible. And when I turned it off I still had problems, enemies would fly about 5 meters away whenever I hit them.