At the beginning of the battle, fight the common soldiers until you're given your orders. Depending on who you're playing as, you will go in one of a few directions, but for this instance, go right. Defeat the officer in the way and continue south until you are asked to capture the base on the cliff.
As you're going south from the first officer kill, you should see a small path on the map that leads straight into the central garrison. Use this and drop into the base, then defeat the officers in there to capture it. Go south from here and defeat any Unit Commanders you come across, then some attack units will appear to the north and south slightly east from your position.
Once you defeat both of those, Ma Chao and Ma Dai will appear in the bases on the western side of the map. They're going to head towards Sun Jian, so quickly head to them and take them down. Also, Pang De and Han Sui will attack the main camp, so after taking out the other two, head there and defeat both of them.
Around now, Dong Zhuo will come and head towards the main camp. He'll need an escort, so head to and follow him, as when he gets close enough Bian Zhang will appear from behind in an ambush. There are a few other officers around, but defeating Bian Zhang will finish the battle.