After escaping the attack in the ninja village, you try taking a ship to find Ryoma.
Prepare Before the Battle
Many of the enemies use axes, and a fair number use lances. For this reason, you should bring units that use swords and magic, as well as some that use axes and bows. Takumi and Setsuna are great units to include in this battle. There is also one bow-wielding enemy, so you might want to bring someone who uses a lance or hidden weapon (i.e. shuriken) to fight that one.
In addition, all of the enemies here are flying units. However, there are Dragon Veins around the field that can cause wind storms, which reduce the movement rate of flying units that go through them. This applies to your own units as well.
Rout the Enemy
You will have to defeat all of the enemy units to complete the battle. At the second turn, an ally unit named Reina appears. Be sure to have Corrin talk to her.
Activate the Dragon Veins to keep the flying enemies from overwhelming you.
Slowly make your way into enemy range to make the enemies come to you. Have Azura sing to Takumi and Setsuna to bring down many enemy units quickly.