Your task at the beginning of the battle is to rescue Zhang Liang and Zhang Bao. Since Zhang Bao is closer, start by heading north and clearing the area around him, including any officers who might show up. Once you do, start heading east to where Zhang Liang is located, making sure to take a pit stop in the area with the stairs to take out any officers there.
Rescuing both officers will make the enemy officers change their course due to the sorcery, which will result in them going to the altar. Go back and defeat all of the officers that will likely be swarming your altar, including Sun Jian, Huang Gai and Han Dang. Zhao Yun will now appear in the northeast, so go there and defeat him, then start moving south down the eastern path.
Note that more Imperial forces will arrive in the northwest and northeast, consisting of Liu Bei's forces, as well as Cao Cao's. You can go and greet them if you want, and after that, start making your way to the enemy main camp. When you reach the garrison with Huangfu Song, he will close the gates, but defeating all the officers around will make Zhang Jiao set it ablaze, opening the main camp. Defeat He Jin to finish the battle.