At the beginning, you can either go south or east. If you go east, you can fulfill the Hypothetical condition for this stage. If you go south, you won't be able to and Guo Huai will end up dying at the end of the battle. Defeat the officer nearby, then drop off the ledge and then choose between the next two ledges after taking out some more officers.
The one to the southeast will place you into the central area of the map, where you will find plenty of officers. The other ledge will lead you into the central garrison, where you will be ambushed by some officers, including Ma Dai. This will eventually trigger an event where Guo Huai gets injured.
As you leave the garrison or go south in the plains area, the supply depot will open and Xingcai will start escorting some wooden oxen to the south. Follow them and destroy all of the oxen, as well as Xingcai and any other officers. There will be another ambush, one of the archer kind, on the southern path, so lure the officers off the ledge above and defeat them both.
As you exit the southern path towards the west side, quite a few officers will appear, so head up to them and defeat them all, with Liao Hua and Zhang Yi being the main targets to open the gates in the western base. As you exit the base to the north, Li Jian shows up and depending on what you did earlier, he may injure Guo Huai or won't try to trick him. After taking him out, Jiang Wei and a whole bunch of officers show up, so defeat them all and then Jiang Wei to finish the battle.
You need to rescue Guo Huai without him getting injured at all, which involves a specific process and set of rules. One thing you should never do is enter the central garrison, as this will trigger the rams Shu will use to break down the gates to Guo Huai's base and injure him.
When the battle begins, go east instead of south, until you happen upon Li Jian. Defeat him and he'll withdraw after stealing some Wei clothes. Now, go south until you see Xingcai talk about supplies, so follow them and destroy all of the little ox carts. Keep going along the southern path and destroy the reinforcements that appear and then Li Jian will appear again. If you defeated him at the beginning of the battle, you will be allowed to take him out again without him opening the gates, thus being able to rescue Guo Huai.