From my experience on normal- shoot until he staggers then keep shooting his leg. He may start to run away, clawing wildly as he does so, keeping shooting his leg and he will fall forward, move behind and hit E when you see the prompt. This is most easily done as soon as you enter the room and can be finished in the first 20s of the fight. If you screw it up suicide and try with a different weapon (ultimately I had the most success with the lightning semi auto gun). Keep in mind you have to kill him once you've pulled the spear to get the weapon- i.e. don't die or you gotta start from scratch pulling the spear again.