In the Cirith Ungol level, you will need to kill Gorbag to advance. After the cutscene, the Orc chieftan will grab a shield. Shatter it with a well-aimed spear, then throw another spear at his unprotected body. He will freeze and not attack you. Move forward and hack at him with Fierce Attack. When he regains consciousness, he will snatch another shield. Again, shatter it with a spear, and launch another shaft at him. Continue this process until he dies and you complete the level.