At the beginning of the battle, start by heading north and clearing the western side where you start. Your goal is to capture the three bases in front of Jianye. When you approach the westernmost one, Gan Ning will appear, so take him out and continue east. Note that Ling Tong will appear south of where Gan Ning did, so it might be wise to go after him, too.
Once you take the three bases, assist in the conquering of the south outer castle in the southeast corner. You'll need to capture all of the bases in the area before the gates to the actual area opens. When it has opened, go inside and you'll be ambushed by Taishi Ci and Han Dang, as well as a bunch of archers.
With the outer area now secure, it's time to head to the main castle. When you get to the main gates, you'll find Sun Shangxiang and Huang Gai there defending it. Defeating them both will open up the main gates and when you get inside, Xue Zong will defect to your side. There will be others who will end up defecting as well, so keep an eye out for them (there are five in total).
Lu Xun will order the gates to be shut and he, Lu Meng, Daqiao and Xiaoqiao all appear, with the two former appearing outside of the castle and heading to your main camp. Defeating the Qiao sisters will open the gates again, so do that and then head back to defend your main camp. Defeat both Lu Xun and Lu Meng, then head back to the castle.
Once you get back to the castle, only a certain path is open to allow you to continue. When you get to certain spots, traps will be sprung and Zhou Yu will appear. Navigate past the traps and defeat Zhou Yu to deactivate them. Make your way to where Sun Quan is and defeat Zhou Tai to open the gates. If you made all five of the officers from above defect, then Sun Quan will surrender without a fight. If you didn't, then you must defeat him to end the battle.