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.
