To be honest I couldn't be bothered with being careful 'n all that. Sent Astarion down there with poison resistance potion, then had everyone follow him (one thing I consider better than many other games is pathfinding) - I think I had to res PC, everyone else made it. One quick rest later and some food everyone was as good as new ready to fight the hag who just stood there 🤷
I also find those vents rather annoying but Larian seems to love the idea.
Also cheated in the crypts in all but the first run.
Judging by the nice chat we had with the dead hag I'm pretty sure we'll be seeing her again and her sister too (who'll probably not be too pleased with us)