After getting medical treatment for your ally, you go to the Ice Tribe village in the hope of reaching Nohr.
Rout the Enemy
There are various villages that you can visit during this battle, but only the northernmost house will give you a reward for your visit. If you visit any of the other houses, they will cause more enemies to appear. However, some of the summoned enemies will drop items, so you might want to visit those houses in spite of having to fight more enemies.
The lake in the center of the map will prevent all of your walking units from reaching the western half of the map. You will have to have a royal unit use the Dragon Vein to freeze the lake if you want to cross, or you can send your walking units to pair up with flying units to take them across.
After you activate the Dragon Vein, more enemies will appear.
Make your way northwest, fighting your way to the boss unit, who uses a weapon with maximum range of 2 and has a high chance of critical hits.