Don't go too far from where you begin the battle, as Liu Bei will disappear near the start and go into the forest alone. Follow him and not far in, a Wu ambush will happen, so defeat the officer and then continue to the intersection. The gates will close and the only path you can go is north. Another ambush will happen the base there, so defeat the officers and then you'll find Liu Bei is currently fighting with Lu Xun.
Defeat Lu Xun to make Zhuge Liang regroup with Liu Bei and convince him to retreat. Follow Liu Bei along and he'll stop every time he comes across an officer, so you will need to defeat them to make him continue his march. When you get to the southwest corner, Liu Bei will stop and you will need to defeat Gan Ning in order to make him start moving again. He is on top of the one walkway, with all of the ballistae, which can't be destroyed but will stop once Gan Ning is gone.
Continue to follow Liu Bei on the path until another ambush appears. To make Liu Bei continue moving, you will need to defeat both Ling Tong and Xue Zong. Liu Bei will continue to make his way to the escape point, but there will be one last ambush in the way before he can finally escape. Zhou Tai will appear and once you defeat him, Liu Bei will have a clear path to escape and finish the battle.