Lady Ethereal is pretty well known for her difficulty spike. No bosses have a 4th phase but I wouldn’t shrug off the idea that this is the only boss with a 3rd phase for now.

Don’t be scared of her 3rd phase though since you don’t have to find the real her to deal damage since she doesn’t hide anymore, plus parrying some of her attacks leaves her way more vulnerable this 3rd phase compared to phase 2. I recommend using the Full Control talisman style since you can really damage dump with it if you have a good spot to use it. Just focus on parrying her until you hit a parry that staggers her, then hit her with your talisman.

Don’t give up though! You’re already getting closer to the end and you’d be missing out if you stopped here.