At the start of the battle, your main camp will be set on fire and you need to track down the officers in charge of the attack and defeat them. Liu Yin is the one located slightly to the west and the other is to the northwest. Once you have defeated both, the fire attack will be extinguished. Zhao Yun will likely have begun his charge towards the main camp, so head over to him and defeat him.
A lot of things will likely be happening at once now, with ballistas appearing in the center of the battlefield, as well as Meng Huo showing up in the south. For now, board the boat in the north and it will stop in the middle of its journey. Use the ballista on board to destroy the three on the shore and once you do, Guan Ping will jump onto the boat. Take him down and the boat will land, so get off and start moving towards Jian Yong in the middle of the map.
There will be plenty of other officers around Jian Yong, so be careful. You'll likely have triggered Guan Yu and Zhang Fei's appearances, who show up near the front gate of Baidi Castle. Likewise, Ma Chao and Ma Dai will appear in the southwest corner of the map. Defeating Jian Yong will stop the ballistas and make your side start to build their own catapults.
Use the catapults to knock down the main gate for Baidi Castle and when you do, go inside. Some rock throwers will appear, which are tied to Guan Yinping, but you need to go around to get to her. On either side are some stairs, which will trigger the ballistas to start firing, which are tied to Zhuge Liang. All before Liu Bei, you'll likely fight Guan Xing, Guan Suo, Zhang Bao, Zhuge Liang and Guan Yinping. Once the final gate opens, finish off Liu Bei to put an end to this battle.