Corrupted Monk Illusion is a major boss in the Sekiro game. You will earn a massive amount of rewards after killing her.
Follow the steps down there in order to defeat Corrupted Monk Illusion.
How to defeat Corrupted Monk Illusion:
The first thing you need to do is to get extra emblems. Then reach closer to the Corrupted Monk and attack using one firecracker and slash her using your sword. This will be a damaging combination, repeat it for four to five times.
Corrupted Monk is equipped with a long sword. When she comes to attack, you just need to step back a bit and block her strikes.
Whenever the opportunity comes, attack her with double ichimonji.
After doing the double ichimonji, kill her with the final blow by striking the neck with your sword.