Dynasty Warriors 3: 4th Weapon Guide - Zhang Fei

This is a guide in order to obtain the 4th weapon for Zhang Fei, in Dynasty Warriors 3.
Weapon Name:
Viper Blade
Base Attack: +47
Element: None
Other Stats:
Attack: +29
Defence: +58
Mounted Attack: +56
Mounted Defence: +57
Musou Charge: +24
Requirements:
Stage: The Battle at Chang Ban (Original Mode) - Liu Bei's Forces
Defeat Xiahou Dun and Xiahou Yuan, and activate the cutscene where Zhang Fei shouts down Cao Cao's Forces at Chang Ban bridge. Supply Captain appears in the south east corner of the map, and proceeds north through the map.
Other notes:
This is a tricky weapon to obtain, not through the difficulty of facing troops, but getting the generals to cooperate with you. In order to activate the required cutscene, one general must approach the bridge while you stand on it. The AI loves to target enemy troops over you a lot of the time, so it can be tricky to get their attention on you as you lure them in.
What makes this doubly annoying is that Liu Bei gets into trouble very early in the battle, being sandwiched between Zhang He and Xiahou Yuan's units, and only gets worse as more troops join them. He can survive for a while, but is not invincible so you are limited in the time you can take to deal with triggering the cutscene at the bridge.
There are two sets of forces that can work for this purpose: Li Dian and Xu Zhu in the south, joined by Cao Hong when Cao Cao arrives, and Cao Cao in the north, possibly joined by Zhang Liao if you kept him alive. The southern troops have Zhao Yun in the way, and the northern troops have Zhuge Liang to deal with, and whichever is killed off first is the most likely direction that you can get the general to approach you from. Letting the enemy troops gain momentum is counter to the normal approach you have for battle, but the way the AI operates gives you very little choice.
Liu Bei doesn't get much protection through this battle, and once you clear out Xiahou Yuan, a supply unit appears requiring you to once again move from his side to obtain the weapon. Clearing up the rest of the troops can be a problem as most will have gained a lot of morale by the time you get around to them, and it's likely most of the peasant units will die off easily too, preventing the alternate win condition with Liu Bei escaping.