At the beginning of the battle, defeat the officer nearby to set up an altar that will summon phantom soldiers. This will open the gate to the north, where you'll find another officer, who will open the gates to the east in the previous area once defeated. Follow this path, defeating the officers in the way, until you arrive at the designated spot.
Here, you will need to take out the officer in charge of the area, noting that the ballistae are active, so destroy those first. Another altar will be placed here, so continue to the north. There will likely be arbalests here, but you might be able to breeze by them if you're fast enough. If the messenger gets to Cao Ren, don't worry, as Zuo Ci will still summon phantoms of Lu Bu and Dong Zhuo.
Either way, defeat Cao Ren and then head into the arena-like area to the west to find Xiahou Dun, so take him out as well. Go south and then west only to have to backtrack because of Sima Yi seeing through your tricks and sending engineers to take out your altars. Go back to the area with the arbalests lined up again and then north into the area with Li Dian.
Defeat Li Dian first, then go north to find Sima Yi, who will sic some pyrocannons on you. Destroy them first, then go after Sima Yi. Once you defeat Sima Yi, the gate on the west side of the area where you fought him will open, allowing you access to Cao Pi, who also has Zhenji with him. Defeating Cao Pi will end the battle.