Even your WoTLK Classic Gold own forces aren't immune to the legion's corruption.
The next step is to head south through the doors and traverse the valley to reach the barrow dens. But just as you, you encounter some of your allies, who warn you that the forest is horribly infested by the World of Warcraft, and they aren't kidding. Every few steps, you'll be dealing with a variety of skeletons, poisoned treants, and ghosts. The ghosts are the biggest of your worries, since they can possess your units and turn them against you.
Each time this happens, you'll lose that unit and will have to kill it. Your heroes can't be possessed, however, so try to attack the ghosts with them and keep your regular units on the lesser enemies. Keep fighting across the forest, and to the southeast you'll finally encounter a death revenant and its guards. Destroy everything, including the revenant, and you'll restore the forest to its original, unspoiled state. Now you can move your ancients through the forest and up to the last mine, which should be visible to you now.
Once you plant your base at this last mine, restock your forces. Make sure you have a reasonable number of all the combat units, along with at least one ballista, and head upward to the corrupted night elf base. There are no enemies here that will give you serious trouble, so plow through them and destroy all buy WoTLK Gold the buildings in sight. Once this is done, move Furion up to the barrow den to finish the mission.