Is Lady Butterfly the hardest boss in Sekiro?

Could you possibly elaborate on why some players might consider Lady Butterfly to be the most challenging boss in Sekiro: Shadows Die Twice? Are there specific movesets or strategies she employs that make her battles particularly grueling? Does her difficulty level vary depending on the player's proficiency or the chosen difficulty setting? And, if you've faced her yourself, how did you ultimately overcome her to progress further in the game?