The boss you're fighting isn't really one to be defeated just through sheer brute force. There's plenty of other methods to deal with him, you just need to use the other tactics available to you.
You can try using items. You could try using Strip. You could let him grapple you and then Resist at just the right moment. There's a handful of different ways to win this fight, and grinding isn't the intended solution
Otherwise, thank you for the praise with the game. Glad you seem to be enjoying it by and large. And hopefully with this advice you can get past that particular enemy and enjoy the rest of what the game has to offer.