Boss battles in Sekiro: Shadows Die Twice are not particularly easy to complete, however they can be made much simpler when finding the right items or weapons particularly with the Lady Butterfly.
As the Lady Butterfly moves very quickly, it would help to have the right equipment that will aid you in defeating her.
Using snapseeds against the lady butterfly will make the fights a lot easier, as this consumable item breaks the illusion of her apparitions.
Although you will receive one snapseed from a samurai outside of the boss room, it will run out very quickly making the fight much more difficult and snapseeds are difficult to come by and can only found in certain locations.
To find snapseeds you will need to make your way towards the Underbridge Valley once in Ashina Outskirts, when you make it down past the Idol Statue and encounter the giant White Serpent.
You will need to stealthily pass by the serpent by waiting in the tall grass for the White Serpent to look away before crossing the valley and using your grapple to get clear.
This process will need to be repeated before reaching the wooden palanquin, where you can hide from the serpent briefly, when it gets close to look inside, you can do a deathblow to its eye that will give you the opportunity to make a run for the exit.
Once you’ve passed the serpent, you will be able to reach the Ashina Castle Gate Fortress and the Idol Statue.
However, you should turn back to where you fought the white serpent and there will be a glowing item on the hillside near the palanquin where you stabbed the Serpent, this bush will allow you to grab five snapseeds which will be crucial to defeating the butterfly lady.
When using the snapseeds, it is important to be close to the apparitions before activating them, as the wave wont reach them if you are too far away.