The fight against Ethel is in my opinion the one fight, where you are better off leaving the fighter (Lae) at the camp, since Ethel is zooming around so much. I have instead Wyll (for Hex) or Astarion (who is better with a bow than Lae'zel) in the group and it worked well.
As someone said: make sure, that the masks are dead before entering Ethels lair, because she summons them and then the fight will become really hard.
The masks I normally fight by sneaking into the dungeon and getting to high ground with as many characters as possible, before they spot me.
I normally start the fight in Ethels hut since by then it is pretty clear, that she is evil, and once or twice had the luck to kill her there. You can then talk to the halfling mask and she can be freed, which is nice. Don't talk to the other masks, they will attack you.
I think you can abuse the killing Ethel in the Hut mechanic, if you want an easier time, but the normal fight should be doable, if you try to keep an eye on the real Ethel and mostly ignore the copies.
