During the opening scene, Kaden and Keaton will both join the party. The objective is to defeat the Vallite bersker in the middle north of the map. However, you’ll also have to deal with the hostile Nohr and Hoshidan forces. There are two dragon veins to the NE and NW corners of the map. When activated this will lower the tide, making the river passable.
The only way to reach the boss is by using strong fliers and cutting through the Vallite forces in the middle or having your ground forces tackle either the Hoshidan and Nohrian sides until you reach the dragon vein. There are also obstacles in the area that you can move and set to block paths and alter the enemy advance. Taking the obstacle and setting it down takes one turn each. Unfortunately, there are a lot of obstacles and walls that even your fliers can’t pass through. Make sure to check your character’s available movement space by zooming out the map view since some of the alleys can’t be seen from the default angle.
You don’t need to be on a defensive here; use the obstacles to alter the enemy’s movement and attack range, especially to protect your fliers and healers. Make sure that you have the enemy attack overlay activated (press X on any empty tile) On the Nohrian side, you’ll have to deal with the retainers Peri and Laslow, while on the Nohrian side, you have to worry about Scarlet.
You don’t have to engage both armies unless your army is strong enough to deal with two fronts at once. At the 10th and 12th turn, reinforcements for both sides will appear. Keep pushing your forces through and take out the enemies along the way until you reach the boss.