Your first objective in the battle is to head to the northwest garrison and capture it. As you approach it, Sima Zhao remarks that it's too quiet in that garrison considering there's a battle. Once you step inside, an ambush will appear, so defeat the two officers to clear it and open the gates back up.
Capturing the western castle will cause some troops to escape from it and venture into the enemy strategist's base, leaving it open to attack. Go there and defeat Lu Kang, then continue through the gate that opened in the west section of that room. Note that there will now be ballistae scattered around on the path leading to the central room, so make sure you destroy those first when you see them.
Once you get into the eastern hallway, there will be spike traps on the ground that will trigger when you walk over them. On the southern end of the hallway will be Ding Feng, whose defeat will deactivate all of the traps encountered. In the next area, you'll have to fight Lianshi.
When Lianshi is done, use the nearby stairs to reach the throne room where Sun Xiu is located. However, before you can enter, Lu Kang appears once again and will be slightly tougher than previously. Once you defeat him, the main gate will open and you can take down Sun Xiu to finish the battle.