My encounter with Auntie Ethel was rather bemusing. I wasn't trying to game the system or anything like that.
I hope spoilers are ok here,

This was during the second phase after I found the correct Auntie out of the mirror images.
As for Myrinna, I figured throwing a bottle of water on the burning cage should do the thing and somehow that worked.
At this point Auntie Ethel decides to mix things up and make herself a copy of Myrinna
Oh boy, they have a combined total of 16HP, so I didn't want to KO the real Myrinna by accident so I decided to put both copies to sleep.
And then my fighter dwarf decides to bonk the less talkative more obvious fake. Lo and behold who was right. But the game decided to knock out the hag for whatever reason and this scary 100+ hp boss who had insane cheat minions in the previous room suddenly went down in 2 rounds of combat. I don't know if I was smart or super lucky.