Allow me to confirm this. I fought Pinwheel early on my first virgin playthrough pre-Sen's Fortress using maybe an Iaito +5 or something. Granted I still beat him first try going in blind but he didn't die in 5 seconds like in subsequent playthroughs. The funny thing about the fight is it was one of the most memorable because I was completely out of flasks halfway through it thanks to the catacombs being such a real bitch so I was getting really low on health towards the end of it. It literally came down to who could land the final blow first. I panicked and hail married a black firebomb FTW before he blasted me with a fireball.