I'm surprise you didn't get one after 50 kills. Thats some pretty ugly luck you got.
My opinion, the best way to get them is to bring a good dragon element Rapid shot bow and shoot him on the head. Bow has good reach whereas other weapons wont reach his head while he's standing besides the Bowguns, Gunlances, and the Longsword with good timing but even then i wouldn't use a Longsword.