After rescuing the war chest from a hidden ninja, you must fight the Mokushujin in a cave.
Defeat the Boss
The boss is in the northeast corner of the map. To get there, you will have to navigate a bunch of caltrops traps and use Dragon Veins to strategically change the barriers in the cave to protect yourself while proceeding.
If you activate a Dragon Vein while someone is standing where a pillar will appear, that unit will just be standing on top of the pillar afterward, and can move in any direction away from the pillar afterward. You can do this strategically to raise barriers but allow a unit to go to whichever side of the barrier is best.
Saizo will be an ally unit in this battle, and will scout ahead, disarming caltrops traps and fighting enemies.
Also, any unit that knows Locktouch can safely walk on caltrops traps without being harmed, and can also disable those traps.
You will also want to open the chests in this map. One contains 5000 G, and the other contains a Master Seal.
As you approach the boss's location, reinforcements will show up from the west and southeast.