Go south at the beginning and an officer will spawn in the first area, who will disappear once he's taken enough damage. The next room requires you to take out all of the soldiers in order to open the gate. In the area beyond, phantom soldiers will be summoned, which can't be damaged at all so ignore them. What you need to do is take out the three sorcerers who have summoned them, who are Zhang Bao, Zhang Liang and Zhang Jiao. Once you do that, the phantom soldiers will disappear.
Investigate the room to the south and a phantom version of Cao Cao will appear, so defeat him to continue. Wang Zhong will appear once again in the area to the east, so head over there and take him down to find out that the real culprit is Zuo Ci, who goes to the throne room. When you return to the previous room, phantom versions of Dong Zhuo and Yuan Shao will spawn, as well as a few other officers.
Kill them all and then Lu Bu will appear in the small room leading to the north. The very large courtyard-like area will have Wei soldiers that will eventually defect, as well as phantom versions of Liu Bei, Zhang Fei and Guan Yu. Defeat them all to make the final gate open leading to Zuo Ci, who will make doubles of himself appear. Defeating the real one will end the battle.