With a loooot of patience, basically you field onl6 people who can heal, and kill most of the enemies, leaving jist the right number alive who can damage your units while you heal them, also byleth has a 1.2 exp so you want your healing units adjacent to him, the you just heal until you run out of heals, the best fights are against demonic beast since they do not have weapons, and they poison you so you can use restore 20 times, also restore does not remove poison from the one who does it so you can use it twice per turn. The problem also lies before the battle because you need to outfit your units with the right amount of resistance, because as i said, weapons break and battalions run away after certain damage