I'm all for too-rare defensive encounters like this where the player is caught flat-footed and can't use their preferred MO. The only point I'll concede is that Isobel's HP and AC were too low for tactician (and that's a common enough problem for encounters on tactician with friendly NPCs throughout the game too). Sounds like that was fixed patches 1&2.
On my first play through she got hit hard immediately and I was scrambling to protect her. I threw up Sanctuaries even using higher slots, considered throwing potions of invisibility but thwarted by turn order, and chugged 2-3 potions of speed; that run was all about saving _all_ NPCs. Every available summons was used to block approaches. Had the Blood of Lathander mace then too, so undead attacks were at disadvantage. Finally, Marcus got pushed off the second floor and decided to run around, saving the day.
Just remembered, you can have your fighter draw OAs! I suspected Isobel would move stupidly, so I had my paladin move beforehand to draw them. It's a valid tactic for high AC/HP characters.
As mentioned, Isobel needs just enough AC/HP to last round one, rest is on player. I suspect Fog cloud/Darkness would help - I wonder about Feign death?
One thing not being discussed is the importance of initiative. I'm playing through Act 1 again and there are ways to improve it. Not a lot, true, but some. And it's already making a difference. Don't overlook +1 here or there.
(In fact, it doesn't look like BG3 uses TT initiative. The variation is too stable. I suspect they use static initiative with a 0-2 variance. If so, that +1 is a bigger shift than you'd think, and +5 is huge.)
