What exactly is your issue with that fight? Did you get surprised by it on a day where you had already used all your spell slots and other limited ressouces?
To me it was one of the easier fights in this game. I did focus Marcus in the beginning and then killed the demons after he died. My battlemaster Lae'zel with action surge did a ton of damage to Marcus in turn 1 so he didn't live too long. Additionally I was using gear that applies temporary hp, bless and blade ward to targets of your heals. So a single healing word on Isobel increased her survivability a lot.
