Meng Huo is nowhere to be seen at the beginning of the battle, plus the Shu forces are attacking your officers and even main camp. Go to each officer in trouble, starting with the main camp, defeat the enemy officer harassing them and learn any information as to the whereabouts of Meng Huo. One of them will point out an allied officer in the northwest section of the center area, so go there.
If you use the stairs that look like a bridge on the map, you'll see Guan Yinping ambush you on the west side with some rock throwers. Defeat her to make the rock throwers stop, then continue to the last ally in trouble, defeating the enemy officer to rescue him. Another enemy officer will appear from the central garrison, which is where Meng Huo is located and fighting with Zhao Yun.
Defeat the other officer, then rescue Meng Huo and exit out the northern gate, only to have some enemy reinforcements appear in the middle eastern area of the map. Go back and defeat them, then continue towards the enemy main camp. At the one garrison, you'll have to defeat both Bao Sanniang and Guan Suo to continue. After that, it's a straight shot to the enemy main camp, where you'll find Zhuge Liang and Yueying. Defeating Zhuge Liang will end the battle.