Izetta: The Last Witch: A False Miracle

Izetta is powerful enough to fight whole armies, but her strength is limited. She can only use her abilities near certain ley lines, and outside them, she's no different than a normal girl. When the Germanian army attacks in a spot far from these ley lines, the duchy is forced into a corner: how can they repel these foes without giving up the secret of Izetta's strength?