When you go talk to her and even Jaheira tells you, she's the reason that magic shield warding off the curse is her doing. What more do you need as a hint for the consequence of letting her get kidnapped? Even the dialogue from Marcus states plainly that he is here for her. Why wouldn't you be interested in protecting the cleric just earlier shown to be the source of the protection dome? Are people just not prompting the secondary dialogues available to NPC's?
I beat the fight before the hotfix patch on Balanced for my first playthrough, running a barb/druid/cleric/wizard comp. Marcus went first and still Isobel didn't come even close to dying because you have 3 characters who can apply CC to Marcus and Shadowheart with her Hellrider gloves applies 50% physical damage reduction to whoever she heals. Those gloves are pretty much impossible to miss at Act 1, since you get them for free for helping the tieflings. Shadowheart can also cast sanctuary so Isobel can't be targeted until she attacks, and on top if you throw an invisibility potion on Isobel, she can't be targeted by mobs either until she takes her turn and drops invis by attacking.
This is just one of the possible trivial strategies to protect her. You can also just burst out Marcus with a high level magic missile spell and giving Karlach a speed potion chug. He won't live past a single turn.
You have no idea that she will be kidnapped before Marcus turns up and once he arrives and you find out he wants to take her you do not know that the game is rigged for her to go down easily and then a cutscene will kick in. You also have no idea that the barrier will collapse immediately if Isobel is so much as unconscious. Nor do you have any inkling that everyone will turn into zombies the instant the barrier collapses.
The game doesn't explain why Thorm's long dead daughter is an alive and well priestess of Selune. Nor does it explain why you later meet her again as an undead at Thorm's side.
As the OP said at the start, the actual fight isn't the problem so providing your 20-20 hindsight tactics aren't to the point. Like a lot of people I walked into this fight and got caught completely off-balance. I lost Isobel but defeated all the zombies easily enough. I rolled back and did the fight again and Marcus didn't last two minutes.
The writing in this game is piss poor - it's the same looooooooooow standard as the crap that Rings of Power managed and that Wheels of Time is currently managing.
