To defeat him, you must destroy the chair at the top of the room. First, get the rocket launcher and flame thrower found during the level. When fighting him, run in circles so he cannot get you (Martyr recommended) while shooting the chair
with the launcher. When you run out of rockets, use the flame thrower. Note: The chair will shoot out blasts of "energy" unexpectedly. Burn the chair from the sides. With the chair out of the way, run in circles while shooting him. If you run out of rockets and gas for the flame thrower restart the game, as the other guns do almost nothing to the chair.