My coop campaign was having a lot of trouble keeping Isobel from being captured and didn't want to have to wipe out everyone at last light in due to the shadows etc etc. We succeeded in killing Marcus but the horrors can abduct her anyway if she gets paralyzed.
Finally, on the 3rd attempt we thought what if we try something way out of the box.
We had one invisibility potion, and Shadowheart had the spell command. Commanded Isobel to flee, and threw the potion onto her causing an invisible Isobel to run off and hide for 3 turns while we mopped up Marcus and the other enemies as fast as we could.
The plan worked and now we have Selune's blessing AND everyone at last light is still alive and well
The Sanctuary spell is an easier way to do this. She never makes any attacks so it's 10 rounds that she can't be targeted at the price of a bonus action and a lvl 1 spell slot.
You have to test the barricade by trying to go through, if it says you can't go there, all is good. I believe you can block the side balcony but not the one Marcus comes through.
What level are you? I did it at level 9 without issue. Marcus was paralyzed immediately and died the first turn, then with everyone in the Last Light, we easily cleaned everything else up.
How did you even get to level 9 at that part of the game? I'm almost 9 and feel like I have done every possible thing in act 1 and 2 except for the end of 2. I was like level 7 when I did the Marcus fight.
I didn't talk to Isobel until I pretty much explored all of the Shadow Lands and some of the Gauntlet. I was putting off talking to Jaheira so I didn't even know about Isobel until I saw her doing her mirror moon fireworks show from across the roof and was like "that looks like a big thing I'll come back later".