Exit the camp and start taking out the enemies in front of it. Get the gates to the central garrison open and then defeat Yang Fu inside to complete the mission. At this time, Cao Cao will begin moving up the east side of the map to meet with Han Sui, so you'll need to escort him there. Also, Xu Huang will now begin defending the main camp and the victory/defeat conditions will change.
Keep following Cao Cao until Ma Dai appears, so take him out and then Cao Cao will meet up with Han Sui. They go into a private room and Ma Chao wonders why Han Sui isn't moving. This will trigger two sets of reinforcements of nothing but normal soldiers on the east side, and it's your job to continue killing them to lure out Pang De. If you fail, then Han Sui doesn't defect and you'll have more officers to kill.
If the plan succeeds, then Han Sui and a few other officers will defect to Wei. Either way, Ma Chao will eventually appear in your camp, so make sure you're there to take him down. Note that you should be here, as it's likely your main camp has been under attack while you were on the east side of the map. After taking out Ma Chao, you will need to take out the ballistae at the first gate.
To get there, you'll need to go up the eastern path and into the garrison there, then take the southern exit, go up the hill and go west before the drop. Destroy both ballista and the ram will break through the first gate. It will stop before the second gate, as there will be more ballista, so use the western path in the area to get to those (take down the officer in the way).
Destroy the next two ballista to make way for the ram to the second gate and once it does, exit to the west. Go north and take out the officer to take over the catapults. Get next to one and use it to bring down the third gate, then drop down and pass through it. Feel free to go and destroy the ballista, but all that remains is to clear the last two areas and then defeat Ma Chao to win the battle.