Your first overall objective at the beginning of the battle is to head over to the floodgates and close them, which will call in reinforcements for your forces. However, there is another problem at hand, which is the possible defection of some of your own officers. Thus, you should concentrate on clearing out the enemy officers in and immediately around Xiapi Castle to prevent your own from defecting.
Once you helped out your own officers, head on over to the floodgates and defeat Jia Xu in order to close them. When you do, start going around and defeating any other officers that you find, with Liu Bei, Guan Yu and Zhang Fei being directly outside of the southern entrance to the main castle.
Start making your way to Cao Cao's main camp in the southwest corner. Defeat Xu Zhu to open the gates and when you enter, Cao Cao will retreat, which will trigger Guo Jia to light the main camp on fire. Defeat him and Cao Cao will reappear in the northwest corner, with him trying to take over Xiapi Castle. Head up there and you'll find Xiahou Dun and Xiahou Yuan with him. Defeat Cao Cao to finish the battle.