After an unexpected battle at the Hoshidan border, you end up in Hoshido.
Rout the Enemy
This battle is relatively easy. In fact, if your units do nothing at all, Ryoma and Hinoka will kill all the Faceless except for the ones on the mountain in the center. However, you might want to have your units kill some of the Faceless to gain more experience.
Getting More Experience
If you choose to get more experience for your units, don't visit the house until after you reach Hinoka and Sakura. Instead, have your units go clockwise, with Corrin activating Dragon Veins along the way, so that you will be in the Faceless' attack range and they will come to you instead of going toward Hinoka and Sakura. Ryoma will still probably kill a lot of the Faceless for you, but you should be able to at least get a little bit of experience by attacking some of the Faceless first.
Once you reach the northwest of the map, activate the Dragon Vein to melt the northwest mountain. In Lunatic mode, there is a Faceless there that will try to attack Sakura. You should try to quickly kill that Faceless, but be warned that it has the Seal Defense and Seal Strength skills, which will greatly weaken any unit that attacks it. Be sure to kill that Faceless before the next enemy phase.
Playing it Safe
If you don't want to bother getting more experience for your units, then at the start of the battle, have Rinkah use her first turn to visit the house just north of where you started the battle.
Have your units move northwest. Don't bother using any of the Dragon Veins. This will only slow you down, because the Faceless will benefit from the increased movement rate and will attack your units.
You should walk between the mountains, going diagonally northwest between the central mountain and the southern and western mountains. You will reach Sakura and Hinoka fairly quickly this way. In Lunatic mode, have Corrin use the northwest Dragon Vein to allow Kaze, Rinkah, and Ryoma to attack the Faceless that is attacking Sakura. With any luck, they will kill that Faceless before it can kill Sakura. In Classic mode, even if Sakura is killed, you will still be able to use her as a unit in later battles. But be warned that this Faceless has the Seal Defense and Seal Strength skills, which will greatly weaken any unit that attacks it. So be sure to kill that Faceless before the next enemy phase.
Once you reach Hinoka and Sakura, only the Faceless on the central mountain will remain. They won't move until you enter their attack range, so don't do that yet. Have Corrin talk to Hinoka to get some Concoctions. On the next turn, have Corrin talk to Sakura. After that, Sakura will heal up anyone who is standing next to her, but only if their HP is below a certain threshold.
Unfortunately, Ryoma will no longer help you once he reaches Hinoka and Sakura.
In Lunatic Mode you should wait until the effects of Seal Strength and Seal Defense have worn off of whichever unit attacked the northwest Faceless, IF any of your units did attack that Faceless.
The boss Faceless does not move until you enter its attack range, so leave it for last. You might want to separate Kaze and Rinkah at this point. In Lunatic Mode, the boss unit will inflict Seal Strength and Seal Defense on each unit that attacks it, leaving them very vulnerable and unable to do as much damage to it. Because of that, you should try to have all three of your units attack it during the same Player Phase in the hope of killing it before it can attack you.