Saltyman | Sekiro - Corrupted Monk Illusion Boss Fight @SaltymanGaming | Uploaded August 2021 | Updated October 2024, 17 hours ago.
Corrupted Monk Illusion only has one health bar but she has the most posture of any enemy in the game.. It recovers really quick as well so you have to get her health down and stay right on top of her. Or just go for direct health damage.

You can also cheese it pretty easily with Firecrackers, Snap Seed, or Fistful of Ash. Snap Seeds will take a chunk off her health and stun her up to 3 times.