In my playthrough with Ethel, I would start by dousing the fire and casting sanctuary on Mayrina. It keeps you from being fooled later on and keeps her from harm mostly, or you can simply pay attention to their AC, the real Mayrina will be the easiest to hit.
I'll keep a wizard with magic missle which never misses to deal with hag copies, ignoring them is a fatal mistake.
During the time Ethel is disguised as Mayrina leaves her vulnerable to the hold person spell. You get one chance to cast it on her depending on how many have that spell to cast before Ethel's next turn. If successful, this will leave her open for a pounding. If she breaks free, cast command on her.
Was a pretty easy fight for me. I did it at lvl 4 before the underdark.
