You can only bring 4 units in this battle. The objective is to defeat Yukimura in the northern part of the fort. There are two chests in the middle of the map and the enemy units carrying their keys are located beside the chests. The dragon veins in the area can be activated to reveal the healing tiles in specific locations of the map.
Once the battle starts, move your units to the center room with the first chest (Killer Lance) and dragon vein. The enemies to the SE and SW will attempt to flank you so heading to the room with the healing tile will help you survive while whittling down the enemy numbers. Once you’ve dealt with the attackers, heal up and move to the second healing tile location. Aggro nearby units to take advantage of the healing tiles then move east to loot the second chest (Wyrmslayer)
After looting the second chest, prepare to move in to engage Yukimura. He’ll be with Saizo and Orochi. Though you’re not required to engage them, they’ll move towards your party once you’re in their attack range. Yukimura will not move until you enter his attack range so you can deal with Saizo and Orochi first just to be sure. When you’re done, engage Yukimura and finish him off to complete the objective.
After the chapter’s ending scene, Kaze, Sakura, Subaki, and Hana will join your party.