During the first battle, the Battle of Xu (on expert level), have Guan Yu defeat Li
Dian and then go attack their storehouse, which is located just past the upcoming
bridge. You get lots of experiance when you destroy their supplies. After you do
that, have Liu Bei lose on purpose to Xiahou Yuan. Repeat this alternating Guan Yu
and Zhang Fei back and forth, until you get too strong so you cannot lose, even if
you try. (Note: When it is Zhang Fei's turn he defeats Yue Jin.)