At the beginning of the stage, go west and after clearing the gate, the bridge ahead will be destroyed. As you start heading south now, an ambush group will appear, so defeat the officers at the base of the cliff, then defeat the fire archers at the top, which will trigger Zhu Yi to flee.
There are two paths here, one that goes underneath and one above, so take the one above. Eventually, you'll come to an officer preparing to use a catapult, so defeat him to take control of it. Use the catapults to get rid of the ballistae, then unite with the other group again.
Clear the area in the south and then Zhu Yi will lead an ambush to the same place, so clean them up. When Zhu Yi goes back to his commander, he is executed and some of his other men will defect to your side. Sun Chen will send out a warning, which is two officers, to anyone else who would like to defect, so hunt them both down and get rid of them before they can meet.
Once you do that, you'll have two new allies on your side. They will open the gates to the main camp, so go inside and start doing some cleaning. When you open up the eastern gates, some more men on Wu's side will defect to yours. This will open up the final area, so go inside and defeat Lu Kang, who surrounded himself with some arbalests. All that's left is to go and defeat Zhuge Dan to finish the battle.