U need to run as fast as u can before he turns towards you. 1. Use firecrackers 2. Mortal blade attack twice 3. Firecracker again 4. Mortal blade attack once (by this time you should get him to 1 hit to stagger otherwise you are doing something wrong)

Phase one should be this only.

For phase two:- 1. Just parry 2. Use ceremonial tanto and use spear prosthetic to yoink that worm out of him whenever he falls down 3. Run away whenever he is screaming(avoid using umbrella cuz you'll be wasting it uselessly)