At the beginning of the battle, go north and once you encounter the first officer, some messengers will appear ahead. Run as fast as possible and chase them down (there are three), as they will tell their allies that you are coming and it will ruin the decoy plan.
Now that you got rid of the messengers, start making your way to the mountaintop. This is easily done by defeating Ju Fu and Chen Zhi, then going through the gates that open. At the top, defeat Zhuge Zhan and that will take care of the enemy pincer attack. Now you can start making your way down the mountain and towards the enemy main camp.
When you make it to the enemy camp, the officers guarding the three gates will rally and become slightly tougher. Defeating two of these officers will make the third fall back into the main camp, so give chase and take care of that one. As soon as you step foot into the main camp, Yueying will appear with some Juggernauts. Take her out and then go into the newly opened area to defeat Jiang Wei.
What appears to be the end of the battle isn't, as Jiang Wei falls back and an extra section of the map appears with more enemies. As you approach the next area, ambush troops will appear to the east, while Liao Hua will commence an archer attack on the cliff to the west. There will be a few officers around, including Xingcai, who when defeated will open the gate to the east.
Ma Dai will appear to the south and begin causing a rockslide, making boulders roll down the path. In the northeast corner of this area is a hidden path that you can use to bypass the boulders and get behind Ma Dai, making it easier to get rid of him. On the path leading to the western section, Dong Jue will make some fire archers appear, halting the Wei progress once again.
Continue past the area with the fire archers and you'll be ambushed in the south-southwestern garrison. Defeat the officer there and then defeat Dong Jue to cease the fire archer attack. There will be two officers outside of the main camp, one of which is Guan Suo. Defeat them both, then go inside and defeat Jiang Wei once again to finish the battle for good.