The objective is to defeat the boss, which is surprisingly, not Camilla, but Flora. To the south, there’s a berserker carrying a Master Seal, between Beruka and Selena. Your team will be scattered in different ships with enemies on both sides but you can pair them up so save space and make them more effective. The icy sea terrain will also allow ground units to traverse on it like normal ground. However, the icy terrain will also reduce their movement range so use that to your advantage when attacking and positioning your characters. Unfortunately, it seems that your units can’t go down and engage the enemies on ice.
Since you’ll have to divide your forces to two, assign a healer on each group, and preferably another powerful unit on your eastern group since your avatar will start in the west as default. It will be easier to take out all enemies near your position since you probably won’t reach the boss unscathed with just fliers unless you power-leveled at this point of the game and got at least +2 weapons. Aerial units will prove their worth once again in this map. With Reina’s addition to your army, you can use her as a potent mobile attacker. Keep her out of range of the enemy’s archers and she should be able to help pick off lone enemies with ease. Keep an eye on the enemy’s attack range and let them come to you so you can have the attack initiative.
At the end of the third turn, enemy reinforcements will appear on the two major ships to the north and south. At the start of the 6th turn, dragon veins will appear on each ship which will allow you to create an ice footbridge that connects each ship upon activation. Activate these veins to connect your allied ships and prepare for your own assault. At the end of the 6th turn, more enemy reinforcements will appear.
Camilla will start moving once you’ve established the long ice bridge near her ship. She’s quite powerful on her own so you’ll probably need Takumi or several characters ganging up on her to defeat her. Continue defeating the remaining forces then launch an attack on Flora. She won’t from her position and will only attack those who enter her range. After finishing the battle, Camilla, Beruka, and Selena will join your army.