Volcano Staff - Zhang Jiao 4th Weapon Unlock - Dynasty Warriors 3 Complete Edition Remastered

Simple enough, Sun Jian and Haung Gai are the easier ones, as they are closest to you when you cross the river and there are only two of them. But Liu Bei, Guan Yu and Zhang Fei are closer to the crate when it appears, which is on a timer. Fortunately you only lose the stage if you are defeated, so you don't need to worry too much about keeping your allies alive.