Don't even try to fight the mutated rancor; your weapons (even your lightsaber!) will have no effect on it. Just use Force Speed and run like heck to the end of the level. Don't waste too much time fighting the Cultists and Reborn; they actually provide a good distraction for the rancor while you get farther away from the thing. When you finally get to the big conveyor belt, there's a control room that will finally provide the means for killing the beast. Just lure the rancor onto the belt and press the button.