When the battle starts, you'll see Sima Zhao and Sima Shi say something about being careful for ambushes, and you'll see two highlighted green spots. Go to the western of the two and Wen Yang will appear, where he'll head to your main base. He cannot be attacked right now, so turn around and he'll follow him to your base to view a cutscene.
After it's over, you'll get a report that some reinforcements from Wu will be arriving soon on the western side of the map. Go there and capture the western base by defeating the officer and then three waves of reinforcements will come. The first one has three officers, the second two and the last will be headed by Ding Feng and have two other officers.
Defeat them all and then head all the way across the map to the eastern base via one of the southern routes. Note that Wen Yang will appear again to attack your main base, but ignore him and concentrate on the other things. At the base, which Wen Yang should probably have appeared in, is unlockable via the gatekeeper. Go inside and defeat both Wen Qin and Wen Yang, then head into the middle of the battlefield.
There will be two officers who should have already left their bases, as well as three still inside. Once you get the word about the northern base, the three other garrisons will open, so defeat those three officers and approach the northern base to make some soldiers defect. They will open the gates for you, so go inside and defeat Guanqiu Jian to finish the battle.
The description for the star is a bit misleading on the stage select as you need to make sure you defeat all other officers on the map besides Wen Qin, Wen Yang and Guanqiu Jian. The best way to do this is to play as Sima Shi, as he is one of the culprits that can ruin this if left to the AI.
Start by going to the left of the two highlighted spots, which will trigger Wen Yang to go towards the main camp. Follow him to view the cutscene, then go after the Wu reinforcements on the west side of the map. Defeat the three waves and once you do, follow the path to the north until you get to a point where you can drop down into the middle area.
Doing this serves two purposes, which will cancel the attack on the east base (it's a good thing) and draw out the three officers in the northern garrisons. Defeat all other officers on the map besides the three you need, then head into the eastern base. If you see Wen Yang mention a pincer attack and then saving his allies, then you're good. He'll have a short dialogue with Sima Shi when you get close, which will be your cue to defeat Wen Qin first, then Wen Yang and finally Guanqiu Jian.