There will be some forces inside of Chengdu at the beginning of the battle and you will need to find the three officers responsible for letting them in. They are located on the southern end of the east and west sides, as well as the path connecting the two. Once they have been taken care of, it's time to get rid of the forces outside of the castle.
Around this time is when Jiang Wei will appear in the northeast and move towards the castle. He should be your first priority, so defeat him and any other officers in the area before clearing the middle path of officers. As you're fighting them in the middle, tigers will appear and you'll need to go to the garrison on the east side.
Once you arrive, you'll see some Juggernauts sitting there, but Yueying will pop up. You must defeat her before you'll be able to use the Juggernauts, so do that and then attack one of the tigers with it to scare them all away. You may have noticed by now that all of the officers that fall under Meng Huo will reappear if they have been defeated.
In order to stop this from happening, you'll need to go to the garrison on the west side of the map. When you arrive, Zhurong will appear and attempt to stop you. Defeat her and then commandeer one of the catapults. However, you can't use it yet, so continue to Luo Castle and defeat Meng Huo who is inside. This will allow you to use the catapult from before to set fire to Luo Castle (you don't need to fire it at all, so just drive it to the castle).
Jiang Wei will appear just outside of the south exit of Luo Castle, so take him out and he'll move into a base east of the castle, which is only accessible by using the eastern path. As you're going towards Jiang Wei, Guan Suo will appear in the northwest, so go and take him outand then come back down. Other than Guan Yinping, there isn't much else standing between you and Jiang Wei, so defeat him to end the battle.