Entering Greyoll's Dragon Barrow - Elden Ring Ep 16 

Black Blade Kindred is resistant to the damage your weapon does. That's why he took such little damage

To be honest, you probably just need to learn the moveset of The Black Blade Kindred. You weren't doing awful damage, you just need to dodge better and you can probably take it out.

Grey'll is actually not Greyoll but I believe they are related