Everyone is talking about weaponising the magic of everyone, but that's a real silly thing to do. If you need to destroy something, get an army together and arm them with guns.
Witches are in scarce supply, and even they will die if an arrow hits them. It's far better to use the witches in the role as support and logistics and teaching. The only reason why the MC can equip his army with guns is due to Anna producing precise metallurgy, the only reason why so many survived the demonic attacks is because of the healing powers of the witches.
Trying to turn each of them into warriors in their own right is foolish.
Would you really want that small cute healer have to touch a nearly dead person to heal them, only to have them be butchered again in front of her eyes? Again and again and again? Just kill them with an army and be done with it. Revenge is something done swiftly, so healing can begin.