For this one you have to be on the Battle for Fan Castle as Lu Bu's forces. All you have to do is take all the bases, including the main camp, before taking Cao Ren down. For the main camp try and lure Cao Ren and Dian Wei away from your main forces so they can slaughter the troops.
For the other Targets, forget the 3 bases outside Fan and focus on the gates, using the usual methods of taking down defense generals, waiting for a ladder to be set up and then clearing the walls of ballista. Try to get the west gate down to about 50% before moving down the wall to the other gate to clear the ballista and any defenders. After they open one gate they then move to the other.
And finally keep your life about 30%, avoiding or taking down Dian Wei quickly helps as he's the only real problem on this level. Just back off to a base when your health hits 50% and you should be fine.