After choosing a path, you help a village that is under attack by Faceless.
Preparations Before the Battle
Before you start the battle, have Corrin pair up with a flying unit, putting the flying unit in front. You will want Corrin to reach Mozu, the ally unit in the southwest of the map, as soon as possible.
Rout the Enemy
Send Corrin to Mozu, but make sure to keep your other units close to Corrin to prevent him/her from getting surrounded by enemies. When close enough, have Corrin separate from the flying unit so he/she can go talk to Mozu. Mozu will join you.
You should level Mozu up, so have someone pair up with her, putting Mozu in the front. However, be sure to have other units weaken the Faceless before Mozu attacks. That way, Mozu can deliver the killing blow, earning a fair amount of EXP.
Once all of the normal Faceless are gone, send someone other than Mozu to kill the boss unit, which is stronger than the other Faceless.