At the beginning of the battle, you'll be blocked off from the rest of your troops and depending which character you're playing as, you will have a set path (Lu Su in this case). Go north and defeat Zhu Ling to open the next area, where you'll encounter some large fans. In order to get past this area, go to the northern set of fans and you'll find a ladder on the west side.
This ladder leads to a platform with Guo Huai, who controls the fans. Defeat him to shut down the fans, as well as clear the archers, then make your way into the next area. Defeat Xiahou Mao to open the next area, where you'll find some enemy pyrocannons that have been activated by Deng Ai. Destroy the ones that are against you and you'll find some that can be ridden by you.
Use them to take out the reinforcements that show up (or not), then head to the opposite end and defeat Deng Ai. In the next area, Wang Yuanji and Zhang Chunhua both appear, but they shouldn't be too much trouble. Defeat them both and then head north, with some arbalests being used by Sima Zhao. Hunt down both Sima Zhao and Sima Shi (who is opposite of his brother), then head to the main camp to finish off Sima Yi.