Play the Battle of Xia Kou on the hard difficulty setting. You must defeat Cai Mao and Gan Ning before Ling Tong is defeated. Go up the left side first; Gan Ning will appear almost immediately and kill Ling Cao, which will prompt Ling Tong to charge. Gan Ning is in hyper mode for this battle -- be careful. Luckily, you will have Huang Gai and Ling Tong also keeping him busy. Then, just go to the center section and defeat Cai Mao, who is nowhere near as difficult. The precious item box appears just north of Cai Mao's position.