The beginning of the battle will have you on defensive duty, protecting the three garrisons in front of where you begin. This is because the fort behind it is a fake and Wei mustn't find out. Defeat the three officers that come to try capture the three garrisons. After that, an Assassin will find out and begin moving along the southern path to tell of the fort being a fake.
Catch up to this Assassin and quickly defeat him. You must now wait and protect the garrisons from any other officers until Ding Feng manages to get to the cliff to launch a surprise attack. At the same time, some Shu reinforcements will appear in the south. Your goal is to go through the middle and defeat the officers there, including Pang De and Zhang He.
Once you defeat Pang De, Zhang Liao will home in on your position, so take him down, too. Continue west towards Wei's main camp, where an ambush will happen in the garrison. Take out the officers and then push forward. Sima Yi will be in the next area, then there will be a big cluster of officers before the main gate, including Cao Ren, whose defeat will open it. Once inside, take out Cao Pi to finish the battle.