Hopefully, you have a save before talking to Isobel. If so, yes, keeping her HP up is difficult. Not impossible. You need tactics. Have Gale (or own mage/sorc char) cast magical armor on Isobel. Keep her healed after each hit she takes. Focus all dps on Marcus. Use haste potions. After Marcus is dead, rest of the house and your own crew can take out the demon thingys.
And I recognise the feeling that fights can seem impossibly hard, but really... it's mostly learning to play to your characters and companions strengths l.
