I found a way that takes some time but you wont lose a lot of health. When he breaks down the door, run back to the stairs on the side of the room you came in through, and get down them. He cannot see or get to you, so he just walks around to the far side of the elevator. Simply pop up, walk a little ways forward, then shoot him as you run back to the spot, then shot him when he walks away, and repeat. I recommend the flamethrower but anything works.