In the mission where you save the elders from the Hutts, you can easily kill the Rancor by drawing it to the main door, where the prisoners escape though but the Rancor can not fit into. Stand back so that it cannot swipe at you with its arm and throw your lightsaber. It will take a few hits before you kill it. Try holding L as it keeps the lightsaber on him for a while. This is completely useless during the stage for rescuing the prisoners as another Rancor appears, but you at least get the satisfaction of killing it.