I saved Isobel. I positioned my whole party around her as a hindrance for the demon thingies and gave her Invisibility and Magic Armor and Armor of Agathys. Shadowheart healed her when she took damage. I concentrated most of my damage on Marcus and wiped out the rest of the demons after him. An invisible Astarion with his sneak attack moves (and Haste if you have that) or a Sorc or Wiz with a speed potion would be handy to attack two times in a row to dish out high damage fast. The fight can be pretty quick and dirty if the positioning is done right, but it can be a challenge when it is difficult to interrupt the demons or Marcus.
