Magruder is impossible to kill by shooting, as he is wearing armored plating. Instead, use the bow with exploding arrows. Get Magruder to chase you. As you are backing away from him, shoot an arrow at a geyser as he walks over it. The flammable gas coming from the geyser will explode and damage Magruder until he is almost dead. Then, when he is on the higher level, shoot him in the face with the rifle until he yells then get into quickdraw mode. Get ready to shoot the big package of dynamite when he throws it. After shooting a few sticks of dynamite, the roof will collapse on him.