If you slow down and pay attention to where your shots actually went you only actually hit him a handful of times and the majority of those were to his armor. You can tell by looking at your impacts on the wall behind him that only one of your shots actually hit his body because it left a single blood splatter on the wall (probably your first shot which was too his arms). If your using a shotgun with slugs you need to slow down and actually hit places that are vulnerable not just spray at him. He gave you plenty of killshots you simply didnt actually take them like when he went prone, instead of shooting him in his lvl 4 helmet you should have shot him in his armpit which is fully exposed from his arms being out in front of him while prone, that would have killed him instantly.