You start shooting him in the head multiple times, which only knocks off his armor. He growls and approaches you, now berserk and on a rampage. You begin spraying bullets with your BR, which has little affect.
He abruptly collapses, and all you see is the smoke of a sniper bullet passing through his head. You look over your shoulder and see one of the marines holding a sniper rifle. He proudly salutes you, and you nod with approval.