Chapter 9B: Another Trial

You'll find Nyx outside the fort, to the east. To the far east inside the fort, you can find Azura. Surprisingly, you can control her as well but she's unarmed and weak for now. She also carries a Chest Key. The only dragon vein in this map is located to the northeast corner, which calls in an acid rain that will reduce all units' HP to 1. There are also two chests that Niles can open containing Brass Naginata and Rescue.

Don't move Azura yet since she's unarmed and she's quite weak. Don't attack the guards in the entrance for now as well. In the first turn, bring your forces as close as you can to Nyx so you can talk to her at the start of the second turn. After talking to Nyx and convincing her successfully to join your party, she'll tell you that you can break the nearby near her to enter the fort. Do that and create an opening for your party to enter.

After getting in the fort using the hole in the wall, position your forces and send Effie and/or Silas inside, close to the entrance but just enough to aggro the other nearby units. Move east and try to reach Azura. In the area where Azura is confined, there's another breakable wall that you can destroy to reach her immediately. After breaking it, Azura can reach the last unit you used so have her sing to him/her then use that unit again if necessary. You can send Azura, Elise, or your avatar to the dragon vein.

The chest to the north can be reached easily by breaking down the wall near the archer. At the start of Turn 8, enemy reinforcements will appear from the stairs in the middle, within the dragon vein's area of effect. (Save on the turn you plan to use it) Take note that the dragon vein's effect will also apply to your units so make sure you don't have any units in the area. You can leave a paired unit (preferably Effie and Silas) to tank the enemies and draw more of them in the dragon vein's effective area then switch to that unit's support character to have full HP and start taking out the damaged enemies.

Once done, you can now proceed to the boss' group and take them out. You can even have Niles kidnap the boss so you can later attempt to add him to your army. To end the battle, you just need to have any unit you control to stand on the throne and select the Seize command. Otherwise, you can stay a bit and wipe out the remaining enemies to earn more EXP.