I had problems getting her to proc inside her lair, so I have just had to continually kill her in the entryway of the teahouse. I have it down to a science now.
Do whatever your main does best, use Astarion to backstarb, Lez doing push attacks to keep her away from the fire place, while Gale keeps her all greased up, which sets her on fire because she stand near a touch, keeps her from going invisible, and most importantly also keeps her prone. One round and she's down, sometimes two.
![[Linked Image]](https://imagizer.imageshack.com/img924/2541/NOhjy9.jpg)