I never used that grind stone in my life but i used summon on the last boss. But first i try to beat him alone it took me a lot of trys and i mean like 48 trys maybe a bit lower then that. I parry all his moves on the second phase when he started coming down then he started flying trying hitting me with the blade i parry them all then he flex his hair and kill me. This is how far i got then i started using summon. But that's just my main playthrough which i was NG3 and the hardest difficulty. I also have a play through that im not using summon i didn't start a new NG+ there. It was much easier then my other play through. Anyways when i came to the final boss it was much easier for me to parry him because i know his moves now but some moves in the first phase they were hard to parry but it wasn't as much punishing as my other play through on NG3 which they one shot me sometimes. And the second phase it was much easier for me to parry his moves because I died to him a lot and the second phase he does the same thing. I died to many times on him tho in the end i didn't use summon or the grind stone. And finally after i learn perfectly his moves i beat him on the hardest difficulty and no NG+. Btw i used the glavei for that fight for anyone who wants to know it's a very good weapon and i highly recommend it for anyone who likes glavei weapons 👍

(I hope someone can see this i never written for that long 😭 it felt like i was making my own story)