At the beginning of the battle, start making your way to Mt. Xingshi, which is the base in the middle at the south end of the map. When you get to the forest, it will be set on fire and then the gates you went through will close. After that, a Shu assault unit will be sent to take over your main camp, as well as destroy the bridge to the north of it.
Once you get close to Cao Shuang, Ma Dai will appear and drop down from the summit to attack. Defeat him and then continue south, stopping at the gate that's likely closed. It'll eventually be opened and the victory conditions will change, with the battle becoming an escape one now. You need to start making your way up the eastern side of the map and be careful of any officers approaching from behind.
When you get past the little maze-like area, Xingcai will appear, so defeat her and then Cao Shuang will run off again. Eventually, he'll reach an area where rock throwers and a ballista will appear. You need to take down Liao Hua to take control of the ballista, then use it to get rid of the rock throwers.
Keep going to the next base, where you will encounter some Juggernauts and Yueying. Defeat her to make the gates open again and make your way to the escape point, only to have both Guan Suo and Guan Yinping appear. You will need to defeat both of them in order to allow Cao Shuang to finally escape from the battle.