How to get the Tornado Staff
1. Play: "Assault on Cheng Du" (Liu Bei's Forces)
2. Head W from your initial position into the open and when you have progressed far enough, it will change to a cutscene. Also, you will get a message saying Pang Tong's unit has been ambushed.
3.
Defeat: Zhang Ren
4.
Location: E of the initial position of Liu Zhang near the NE corner of the map.