It was the lag, once I was able to damage him I dropped six shots worth 9 damage each plus three heads hits worth 13 each. 9x6 is 54 plus 13x3(39) is 93 damage, plus another burst of 3x9(27) after, which is well enough to take his shield down and be finished with a melee. [spoiler]i know, I just inspected the shit out of the clip. [/spoiler]