For the beginning, just keep going forward until you come to the designated spot, only to have the gates closed and an ambush happen to the south of them. Defeat Man Chong nearby and then continue north to find another ambush. Continue forward to the seemingly dead end, which will open up the next area that has an ambush waiting for you.
After defeating the ambush, go north and follow the path through the northwest door, which will close with an ambush behind you. Defeat the two officers there and keep going to the main hallway where Jia Xu is located. When you get to him, yet another ambush will happen. Clear everything out, including Jia Xu, who will withdraw. Go out the eastern gate and meet back up with Guan Xing.
On the easternmost path will be Sima Zhao and upon taking him out, the bridge behind him will lower. Go south and then west to encounter a lot of officers, including Sima Yi, Wang Yuanji, Sima Shi and even Jia Xu, who has reappeared. Defeating Jia Xu will make the main gate to Cao Pi open, who has Zhenji by his side. Take out Cao Pi to finish the battle.