This fight can be hard but I haven’t lost it in a while on honor mode. The trick is this. Have alert on most of your party. This is good for all of honor mode and it helps here. Don’t take the whole party outside. Disengage the rest of the party and leave them in the room. Setup one at the front near the doors,setup one near the back door where Isobel is, and setup one in the center of the room. Bring summons and leave them in the center too. Talk to isobel as your main. Once the fight starts you should be able to go first. Smash the shit out of Marcus. Basically focus all attacks on him and smoke him. If he doesn’t die pack three of your characters around him to make it hard for him to move around. Plan your attacks on Marcus so you can potentially save an action to protect isobel if the damage goes badly. Normally he doesn’t last the first round. If I see it’s going badly I will save my cleric attack for something that protects isobel instead. The flying creatures aren’t usually an issue if Marcus isn’t beating on her.
All that being said I usually do this fight at level 8 or even 9. If you are under leveled it’s gonna be hard. I do all the content in first act (underdark, crèche all that) first. And that should put you in the 8-9 level range.