This is one of the harder characters to unlock. What you have to do is keep all of Musashi's generals alive, and activate the flood gate.

The hard part is that Musashi's generals are pretty weak, and will die easily, so the first thing you should do is aid them. Kill all the officers in there area except for 1. The one you Don't kill is the one on the center right. The reason for this is because if you kill all of them you won't be able to activate the flood gate, because there will be no reason to.

Once you have cleared out all of the enemy officers around Musashi's officers. Go back to Musashi, and aid him. Clear a path for him to the floodgate, and kill Lubu to activate it. After that you can head for the enemies leader. Be careful of the ambush team on the eastern open field.

I did this on easy difficulty. It doesn't matter what difficulty you do it on, I just wanted to complete it faster.

This level is... Shu Story - Chapter 6-X (less info)