The first time Auntie Ethel fight was one of the 4 fights I ever lost in the game, and I still see it as one of the most annoying fights, even after more than a dozen runs. But the fight is relatively easy at lvl 4 if you are a bit used to it (so, second run onwards), and such prepared. I would never do it with lvl 3.
Have your chars well rested and fully loaded. At lvl 4 the redcaps are very easy, I ususally kill them just with crossbows and cantrips and perhaps one or two aoe spells. After the more difficult fight against the masks I leave the Hag's house and long rest. I climb down to the cave and short-rest before the fight, to get back the hp I lost in the poison clouds.
I would advice to take Wyll instead of Astarion, except you have an intense relation to the latter. Wyll is much stronger in this fight (and generally). Send Wyll's invisible imp onto the bridge towards the back of the cave. Use Magic Missiles from Gale and the imp (who can fly) to attack/disguise the copies of Ethel as soon as possible. Quite often you can remove all copies the first turn they were created, especially if you are playing with Karmic Dices (the default setting). If you want to back up, take Scorching Ray on Wyll, another multiple ray spell. Take out the fire on the cage soon, best with the Rain spell from Raindancer staff. If Ethel copies Mayrina, attack the copy which shows less hit chance. The rest is history.
It's disappointing that you let such a monster live, but to each his/her own.
