Load up your rocket launcher before you go in the electric chair room. Run to the middle of the room, avoiding him. Run around in circles with the Launcher pointed at the chair. Keep shooting it while you run in circles. Make your circles just big enough to avoid him and do not accidentally shoot him at point blank range or you will lose a life. Change your direction when he changes his. The chair should explode after awhile. You can then just shoot him at your leisure or slice him up a bit before regenerating your health. You should be able to kill him without losing a life.