Defeat the waves of Elite Orcs carrying LEGO Banners. Their Banner icons appear on screen and are crossed off when defeated. You will have to fight normal Orcs as well to get the Elite versions to appear.
The volcanic eruptions then cause Morgul LEGO blocks to emerge from the rock. Attack them with Elendil who wields the magical sword Narsil, which can destroy Morgul LEGO objects.
Defeat a final wave of Elite Orcs