At the start of this battle, you'll only have your avatar and Felicia/Jakob. The map is small and there are no notable features like dragon veins or treasures. Pairing up in this mission is not recommended since you'll need Felicia as a separate unit to heal and provide secondary attacks.
Reveal the faceless' attack range by hovering on their sprite and pressing A. You can lure faceless by stepping on their farthest reach while keeping Felicia/Jakob in a safe distance. When engaging the faceless, you can deal more damage and sustain less damage if you transform to a dragon in Combat. Use that to your advantage.
At the start of the third turn, Silas and Elise will arrive to lend a hand. He has excellent defense and attack, perfect for tanking the faceless. Keep Felicia and Elise behind to provide healing and ranged attacks. At the start of the 4th turn, Arthur and Effie will also arrive but that won't make the mission a lot easier. After their arrival, all Faceless will move towards your position so pick a direction where you want to go (north or south) and push your way through the enemies to avoid getting overrun. The faceless will also target your healers first to you have to protect them at all costs.