Kill the other guys, strafe in a circle around him and roll as much as you can when he opens fire with the coffin gun, as he flips over it (or maybe it's right after he flips over it) start firing at him with everything you have. He'll stagger and take serious damage from your bullets as he does. When he regains his balance stop shooting, start circling him again and reload. Use the double pistols and don't bother shooting any other time than what is listed. He won't take very much damage if you do.