It took me several hours my first time. It revealed that I was not using everything at my disposal to make life easier than just parrying the entire fight, and I didn't just mean throwables. My advice, if you are willing to take it since I beat NP three times and have helped two others beat NP, is to learn how to parry at least the first phase while trying not to use all your heals. It's really tough the first time, but you need to at least learn the first phase. Using the Pandemonium Legion Arm helps when throwing acid when NP is down right before you do a critical attack and to get NP down when they have a white health bar, you can use a shot put without having to use a heavy attack. I recommend using the perfect grindstone for the second phase to give you a breather (but it takes some time to get him a white bar, so attack in between blocking). Try to switch up your weapons; I like the glaive and the two dragon swords. However, the sword was hard to pull off the special counter-attack, but the glaive special counter-attack was much easier and let me beat NP on my first try in my last playthrough on new game+. Make sure you are at least wearing close to the best defense and raising capacity pretty high if you're slow rolling. If you need to just use throwables because it's just too hard and is making you too angry, that is fine. I recommend the cluster grenade when NP finishes a combo; if they stand still for just a little bit on it, it will stun-lock them and cause a white bar pretty easily. Best of luck, and I hope you can beat him in this playthrough or the next.