Up your characters' speed. You need to kill her fast. This is good general advice. Combat speed is crucial in this game.
Get a Seraph with the Valkrie class. They can hit the back row with a melee spear attack (Holy Lance). Also its a Light attack which she is vunerable to.
If you have someone with the Templar class they can silence her with Righteous Fire for 2 turns - no spells.