There are a couple of options that are pretty much hag-killers:
If you don't mind losing all of Ethyl's items:
Sneak up (if possible) or just maneuver and push her into the pit. It's an insta-kill, but you lose some closing dialog with her and all of her items.
If you want to just kill her:
There are two ways to get wyvern venom (a 7d6 poison). If you let Nettie poison you and then manage to convince Nettie to provide an antidote, she will offer the venom. Or complete the rescue the druid storyline - there's another dose in the treasure room they open up for you. Two attacks with wyvern venom will 1) take away 2/3 of her hitpoints if she succeeds in both saves; 2) bring her down under 20 if she succeeds on one but not another (this also generates an interesting offer from her); or 3) flat out kill her if she fails both saves. Note that you can sneak up with Astarion and get both attacks in a single sneak attack round.
