On the last section of Amon Hen, you will have to kill the Nazgul and his Fell Beast (the Ringwraith and his dragon.) Go forward from your starting point and slash at them with your sword. (Beware the weird green mist, it can poison you badly!) After a few hits, the Nazgul should fly away. Run around, dodging his shots. He will become vulnerable after a short period. Attack him with your bow. Repeat this several times and Legolas will kill the wraith during a cutscene. You will complete the game after this victory.