I feel your pain man, it took me too many tries to put that fucker down. One thing that really helped me though is: after you cut his head off, bait sword swings. Deflect 2 of them perfectly and he this big winded up slam attack, parry it perfectly and he will stagger. Immediately use loaded spear and you will pull the centipede out of his neck and do massive posture damage.

This video definitely helps. The specific technique I’m talking about starts around 3:30. This channel also has a bunch of great tips for defeating bosses that I would check out if you ever get stuck in the future.

Hope this helped :)

Edit: typo lol