As Gandalf, you will encounter many Uruk-Hai. You will easily kill them with your attacks. When you reach the part where Ents are killing Orcs, you will join the battle and must wait until 75 Uruk-Hai are dead. After this, your journey will
continue. When you reach to group bunch of towers, destroy all of them by shooting your ranged weapon at the explosives under the towers. Once they are destroyed, you will keep going on until the end. At the end you must help destroy the wall that is blocking the water from coming out and wiping out all of Isengard. Kill any Orc that approaches by the Ent and let him break the wall. Once he does this, you will have finished the level.