Ma Su has stationed himself at the top of the summit, but this has caused him to be surrounded by Wei's troops. The only way up to where he is located is the eastern path, as all other routes have been blocked by boulders. As soon as you set foot on the unblocked path, two enemy officers will ambush you and you need to defeat both.
Keep moving along and at the end of the path, some ballistae will spawn. Destroy them and the officer, then move along the center road. Your objective at the moment is to go to each allied officer and defeat the enemy one that is fighting him. There are three that are in trouble and upon defeating that officer, they will retreat back to the main camp.
Once you rescue the three, continue climbing up the mountain and when you pass through the next garrison, it will close and ambush troops will appear. Take them up and continue to where Ma Su, who should have Zhang He closing in on him now. When you reach him, take out the officers harassing him and that should rescue him.
It's now time to escape with Ma Su, but things won't be as easy as just running away. Wei's secondary force will be hot on your heels and there will be plenty of ambushes on the way, including Guo Huai and some other officers. Keep going until you reach Shu's main camp, where all you'll need to do is defeat Sima Zhao and Sima Shi to make Ma Su escape and finish the battle.
|In order to unlock 12H-X. Battle of Chencang, you need to rescue Ma Su before he is trapped. You need to be sort of fast, but if you follow all of the pre-scripted events leading up to Zhang He trapping Ma Su, you should be fine. When you see Zhang He mention something like "They're trying to rescue these eyesores," then hurry up and defeat all officers up to where Ma Su is located. When you get to him, you have to defeat Zhang He and Cao Xiu to finally rescue him.|