This fight annoyed me as well. She was doing okay for herself at the start but she just stands there getting wailed on, casts next to nothing of her own, and then drops like a sack of bricks.
Party was sorc, thief/ranger(astarion), barbarian(karlach) and cleric(shadowheart). SH has no healing spells prepared, I never needed them. I had all my spells as well, but Isobel still drops like a wet paper bag.
