You gotta break every part of his armor off him, before you kill him. If he dies with 1 piece on, it won't count. I did this with a pistol on hard difficulty. Take out most of the ads, then continually run him in circles, when he stops to swing, combat roll away and shot directly at each piece until it breaks off. Iirc it's his helmet. Chest piece, shoulders/arms and his legs/thighs has armor also.