Another person who never managed to beat NKG here. Regular Grimm is tough but manageable for me. Once you recognize his telegraphs, they're easy enough to dodge and counter, but if you mess up or lose focus for even a second, he'll punish it. NKG takes that and just cranks it up to eleven. There are no breaks, keep moving or you die, forget healing unless you can stun him. It's more mentally exhausting for me than anything else.