Go to the Amon Hen level. Once you get to the part where you're fighting Lurtz(the orc/urak-Hai captain at the end of the level that is seen killing Boromir), shoot him two or three times with arrows/throwing axes. He will then grunt, throw down his bow, and pick up a sword. Wait untill he tries to attack you then block. That will temporarily stun him. While he is stunned, attack him. Repeat this untill he is defeated.

TIP: Get behind a statue when he tries to attack you. While his sword is stuck, you can attack him with sword/axe or arrow/throwing axe.