There are three stars from this battle and it is best to play as Dian Wei. Not far into the battle, a fire attack will occur that will severely limit the health of Cao Ang and Cao Anmin. If you fail to save them, then the Battle of Xiapi will unlock. However, if you would like to save them, then you will need to be very quick in getting to them and defeating the officers that are harassing them.
Playing as Dian Wei seems to make it much easier for him to survive the battle, although it's not entirely clear the path to follow. It seems that rescuing Cao Ang and Cao Anmin might be a prerequisite. Once you make it outside the castle, fulfill all of the missions given, making sure you try and defeat everyone yourself.