I have found two possibilities:
1) You kill Ethle before she could enter the lair. For this you have to put your group during the conversation around her and kill her before she escape after two rounds.
In the first round she uses a Invisible potion so you to use a potion/spell which causes an areal damage.
I always do it in this way because it is "easier".
2) You sneak with one character til to the point where Mayrina is caughted. Then you go to the door to Ethel's room. There is a teleporter which brings you back to the swamp. In this way you can move the whole group to Ethel with being in contact with enchanted people.
For 1), I seldom do enough damage within time to eliminate her. I've fought her what, four/five times? Achieved it only once. Think you need Astarion and Lae'zel outfitted and buffed (wyvern poison?).
And 2), I was clever and did that, then Ethel summoned them to the fight! Had to kill a couple of them ASAP, emphasis 'had to' not wanted to, and they had already wrecked the party.
Only in my first play through did I shove Ethel to her death. I thought back then that the fight was so easy; haven't managed it ever since.
