When the battle begins, go forward and drop down to the large room on the eastern side. Here, defeat all the officers and this will activate the wind generator. Once you do that, find the stairs in the southeast corner of the room to find a hallway of statues. Defeat the officer and then find the path that goes onto some balcony-like areas with ballistae.
Defeat the captains to seize the ballistae, then use to hit any nearby captains/officers to seize those ballistae. If you fall off the platform, there are stairs on the west side of the room, as well on the east side. You can use these to reach some of the enemies that let you seize the ballistae.
Once all of the ballistae have been taken, as well as the Wu forces have been eradicated, you can head to the area outside the castle. You need to take the two garrisons in front of the Wu main camp. When you do that, the commander of the Wu forces will be mad and open up the gates to the main camp, so defeat him to finish the battle.