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 🤷
Sounds like a mess to me.
