Eight inch thick skull
I'm sorry but its only bone and most armor piercing shells would go right thru it especially when being fired from what looked like a 20mm cannon at almost point blank range.
Try it sometime Mythbusters!
I agree. I thought they were using a .50 cal from a roof top. That might be questionable since I was in the army and .50 rounds, like many could do some weird things, like somehow end up going through targets sideways.
That said, you don't just fire one round. They could have fired enough small arms to blind and eventually kill him. Anything under .50, aim for the eyes, .50 and above, maybe kill shots in the head.