Boss: Kumagera, can be captured.
Leo will join you in this battle. There are no dragon veins in this map and there are two chests to the north and the middle portions. There's a shrine maiden carrying a chest key for the middle chest so you'll just have to send Kaze or Niles to open the chest to the north. The northern chest has a Seraph Robe while the other one has a 10000g. You'll have to rely on your tanks to take the brunt of the enemy attacks due to the limited movement space. Watch out for your enemy fliers since they'll either target your archers or spellcasters and your healer.
Your objective is to defeat the boss, Kumagera. Leo will also join you in this battle. Kumagera can be captured so make sure to bring Niles along if you want. To the south, you'll find Keaton. You must bring your avatar to him so he can approach your avatar and get recruited to your army. Keaton comes equipped with a Beastrune giving him access to a much tougher form that can also deal bonus damage to enemy fliers.
Keep moving your forces along the boats. Leo can be an excellent addition to your force because of his considerable damage output and mobility. Like before, always check nearby enemies' attack range to avoid giving them the attack initiative if you can avoid it. At the 6th turn, a paired Sky Knight and Kinshi Knight will appear from the NW. Work your way in eliminating the remaining forces then Capture/ defeat Kumagera to complete the mission. During the chapter's ending scene, your avatar's Yato will transform to Grim Yato.