In this video I explain how to easily trivialise the Aunty Ethel fight on Tactician Mode, I haven't killed the mask wearers (use protection from Good and Evil) and wear the masks down to the area you fight Ethel before you set up your group composition.
Doing this saves the Masks from you engaging with them in combat to begin with it also prevents Ethel from summoning them during her fight and therefore forcing you to kill them.
when she starts to bargain (if you let her), you can either take the hair (dooming the masks) OR you can just kill her and free them, choice is yours.
Requirements -
Silence Aura (SH has this early in game) - she's light spec for me, can't remember if that makes a difference
Grease - either spare bottles of grease and/or the ability to cast it
Step 1 - group hide once past all the traps to set up group members
Step 2 - Split up group and position each character where you need them (I like SH down below as you can see in the video)
Step 3 - throw or cast grease to make Ethel prone (the person throwing/casting will be the one roped into Dialogue, it's best if they are the weaker one of the bunch) - Don't exit or go through the dialogue just let it continue while character swapping
Step 4 - cast silence aura around Ethel to prevent her casting/calling the masks into combat
Step 5 - use a ranged character or someone like I have here, Karlach, to throw stuff and damage ethel whilst remaining combat free - this is where the fight becomes trivial
Step 6 - IF you want to bargain with her for the Ability improvement you need to remove the silence (Silence also prevents sound/speech)so remove the silence at around 20HP, if you're concerned she might die just pass your turns until she initiates dialogue
Step 7 - MAKE SURE YOUR HIGH CHARISMA character is speaking with her during her bargaining (the closest character will initiate dialogue)
Step 8 - pick whatever answer you want but for the intimidation/persuasion is a 20 Charisma check so make sure you can pass it
Step 9 & 10 - profit?