You have to safe both Cao's, done it once only managing to save one and it didn't work. Basically you have to have them both state they'll protect Dian Wei, for the sake of it I then completed the ambush since it's the quickest way to get the gate open.
Also try it on Easy or Normal because lord knows those lemmings die nigh instantaneously on hard and above the second your morale drops below half which given it's 4 first 20 will be happening almost at the start.
In case you didn't already figure it out and to save you the topic, to save Guo Jia you just have to follow his plans.
Capture the bases surrounding the main camp. Guo Jia will split up the team; you'll follow him in the east to ambush the head honcho (just typing this from memory so I can't tell you the names... sorry). Anyway once he's down his brother will be infuriated; take the brother out. Reinforcements should arrive a bit after his death and Guo Jia says that he'll take care of them. You need to kill the reinforcements and that should secure his escape.
EDIT: Well I feel dumb... just saw that other topic... oh well