Your first objective in the battle is to capture Fu Castle, which is only open from the west, so go that way and defeat the officer inside to take it. When you do, Ma Dai and some other officers will appear to the east and head towards the Wei main camp. However, you can't use the shorter of the two paths unless Jia Chong defeated the officer in the base, so you'll have to go back the way you came.
Ma Dai withdraws once defeated, so continue on to Luo Castle, which appears to be empty for now. A cutscene will occur inside with Liu Shan appearing. While you're fighting him, Jiang Wei and another officer will show up, so defeat all three. Once you step foot out of Luo Castle, Yueying will activate siege weapons all over the map.
Your objective now is to go east and take out Yueying to stop all of the siege weapons. When you make it to the base where Yueying is, Guan Yinping and two other officers appear, so take out all three to open the base, then defeat Yueying to deactivate all of the siege weapons. Note that you'll likely need to go back to Deng Ai and clear the area around him, as he'll probably have a good number of officers surrounding him.
Once everything is clear, start making your way to Chengdu Castle. When you get to the front of it, Ma Dai will appear again, leading some cavalry. Defeating him will open up the two paths on the east and west sides of the castle. In order to open the inner gates to Chengdu, you'll need to defeat Guan Suo on the west side and Jiang Wei on the east side.
When you do open the inner gates, defeat the officers on each side if you want, then go to Liu Shan. Xingcai will also be there, guarding him, and if you defeat her first, then Liu Shan will rally become stronger. Defeating Liu Shan will end the battle.