Your primary goal throughout the battle is to make sure Liu Bei doesn't reach the escape point. If you go north at the first fork, there is a normal officer in the way, but the bottom path has Zhao Yun, who is slightly powered up. Either path will lead to a second officer, which can be bypassed, as the next gate opens by taking down the gatekeeper.
Before you jump down, note that this section is the same as the previous one. If you do jump down, Zhang Fei will show up to the north and he's slightly tougher than normal officers; defeating him will let you bypass a few extra officers. However, going the normal path will have a few normal officers and this path is a lot longer than the previous one.
Once you get to the next area, a fire attack will happen and you will need to defeat Chen Zhen. Getting to him can be a bit of a problem, as the place is a little confusing. When you get to the area that's on fire, there will be a open path to the left that you can use to eventually reach Chen Zhen. Once you defeat him, the gate from before will open, allowing the rest of Wei to join in.
Continue on, going on the long path going south, where you will encounter some ballistas. There are only four, so destroy them all and or take down Liao Hua to make them stop firing. The one path in the next area is blocked, so you'll need to head across the middle area, where you'll find plenty of officers in the way, including Guan Yu. When you do get rid of Guan Yu, the gate will open up and the enemy will reveal a ladder.
Climb this ladder and go into the next base, where you'll find Yueying, along with some of her Juggernaut siege weapons, which should be easy enough to destroy. Past her will be some more officers, including Zhuge Liang. Once you reach Liu Bei, you'll want to hear him say something along the lines of Guan Yu not being there yet. If you hear it, then feel free to defeat him, although the battle won't end there. Guan Yu, Zhang Fei and Zhao Yun will all show up and once you defeat these three, the battle will truly end.