Go forward on the only path at the beginning, defeating any officers in your way, with the first two being Li Dian and Yue Jin. Continue along, defeating more officers, until you run into Dian Wei at the second gate. He runs before saying anything about Diao Chan, but the gate is now open.
In this area, go from ally to ally, defeating the officer and trying to obtain some information about Diao Chan. The first one is fighting with Xiahou Dun and Zhang Liao is fighting with Guan Yu, with the rest fighting generic officers. There is one officer you need to drop in from some stairs nearby, who when rescued, will stop nearby ballistae.
As you get close to Sishui Gate, ambush troops led by Zhang Fei will appear, with Liu Bei also close by. You must defeat both of them in order to make Sishui open. Once you pass through Sishui, you should see a bunch of arbalests, so destroy them all before concentrating on any other enemies.
You must defeat Cao Cao and Sun Jian, which will make the enemy main camp's gates open. Once they are open, go inside of there and take down Yuan Shao. With his defeat, both Lu Bu and Diao Chan will appear in the nearby base. When you rendezvous with them, they will defect and you'll have to defeat them to finish the battle.