Hi,
So you probably already know why you've clicked on this here guide, so Ill try to keep this short
Simply put, I wanted you to know that this is (kinda?) a cheese for the boss known as Lady butterfly, found in the Hirata estate, just following the checkpoint at the audience chamber.
I have (accidentally) found a way to force lady butterfly into a stun-lock using a combat skill, in which i was able to damage posture and, most importantly, damage. How did i find this? Rage, of course, how else?

Before we get started please note that, while this is humongously helpful, you arent invincible. She can still get herself out of the stun-lock fairly easily so while you can dish out tonnes of damage to her in one sitting she still has the capabilities to kill you instantly, especially during the seccond phase.
Also, since I didnt have ANY reccording devices on during the fight, there was no footage of me doing this -- so you will have to take some reference photos as my attempt to explain this as best I can.
Also note that, while this is an easy way to kill this specific boss I have no idea how this will work with others. Also to note it is Imperative That you learn the specifics to deflecting bosses. While it may not be necessarily important in this case, in most cases deflecting will save your life many times.
Great, time to start!

Before we get started, we need to get to know the shift key.
Yes, dodging will be a major factor in this battle. We also need to get to know the space key for a couple of attacks too. Parrying and blocking is not a major factor in this boss fight, and is mostly just there to block projectiles.

We also need a certain combat skill, which is called 'Nightjar Slash' which is essentially...

...That. I think you know how the boss is going to turn out.

In order to get this skill, we have to go to the beginning -- in order to get that specific skill tree. More specifically the prothetic arts skill tree. By this point you may already have it -- but in order to get the skill tree you have to defeat another boss (Yippee...) in order to get the Mechanical barrel.
Who do you have to beat. Well he screams his own name when you first face him. It's gynobu onwia (I think is the spelling) I'm not going to go into how to beat him so good luck.

If you are continuing this i'm gonna assume you beat him. Once you get the mechanical barrel go back to the sculptor and give it to him. You will then be able to use prosthetic arts. Now its time for a bunch of grinding. This shouldn't take long however.

(The one that is highlighted is the thing you need)
Once you get that, you're officially ready to face lady butterfly!

Right, so I will get into how you can get started.
As it begins, she may do a sweeping attack following throwing some shirukens towards you, this is perfect to get the spamming started. Simply hold right click (or block) and spam left click (attack) repeatedly. This not only damages her posture, but as she begins to attack you, you will hit her -- stunning here and damaging her vitality.
However there are chances she can break out of this stun-lock.

If she performs a parry/perfect block (As seen in the image below) She may follow-up with an attack, which can be devastating. She can also however, jump up in the air. If she does this, take no chances, back away and wait for that dreaded danger sign. This means she is going to grapple you. Shift-dodge out of the way and then follow-up with a continuation of right and left click spam. Keep your healing items ready because you may need them, however i advise you keep their use to a minimum until the seccond phase. There, they will be most needed.

If you notice a danger sign, but she is not in the air, immedeately jump away. She is preparing a slash attack which can totally devestate your health early-game. It is pretty easy to continue stun-locking her afterwards.

After stabbing her *ahem* fatally, she will vanish. Is it over? Nope. Now the real fight begins.
Uh i dont have a picture for that so...

So we made it. Prepare to die mostly on this phase.
As she says something like "Impressive boy" Move up to the statue. There you can catch her off guard as she jumps down it and continue doing a stun-lock.
Soon after however, she will click her fingers. this means ignore her and run away. Illusions will begin appearing. Ignore them, run around them in a circle. Make sure they are in the middle of the arena.
Sometimes lady butterfly will join the fray, sometimes she stays up in the air throwing crap at you, either way keep running.
She will click her fingers together again, go to one of the pillers. Take cover and attempt to make the projectiles hit the pillar instead of you.
Speaking of which, as far as i know, they are unblockable. So theres that.
This is why i said to keep healing item use to a minimum. This is where you will most need them.
Continue stun-locking her. At times these projectiles will attack you. If you feel as though you are heavily damaged, shift-run away, and heal yourself and then initiate the attack again.

After a while she will click her fingers again, this time the pillars are on fire. This changes nothing, hide behind them once the illusions are gone. the slash attack and grab attack are still present in this fight so watch out.

Eventually with some good dodging and cold-hard luck, you will beat her and stab her twice for a
SHINOBI EXECUTION

If so, congrats! You win!

So, that's it.
Hopefully this will be of use to someone who is indeed struggling with this boss.
And who knew that acts of pure rage can come of with actually good solutions. For me at least.
Please note that experiences may vary -- you may have alot more trouble than I. Or, less trouble, if so good job.

Also note, this is my first guide -- so if this isnt up to par, It's fine. There's ought to be loads of grammar mistakes so, uh, sorry bout that.
Let me know what you think -- is this actually good? or did i just flunk out on this boss and won out based on luck?

Anyhow this is probably the only guide i'll be making so, Cya.