I suspect you can'T get this title because I don't see how you can get kills on him outside of the questline and i just killed him 20 times (once I had killed him i forfeited the quest and did it again and again, like you would to get knowledge to S) and got no titles. I had already done this quest a bunch of time throughout the years too so likely killed him something like 26 times total maybe. There is as far as I know no other way to face this character outside of this quest so... Probably a title you can't actually get? That or somehow the game counts only one kill per character or something.