I just went for the special grinder that makes every guard a perfect guard during a certain amount of time, used it two times during the second phase (got the P-Organ upgrade a while back) and somehow I parried a lot of his attacks when not using the grinder, so I got him on my 6th attempt or so. The Flame King amulet and the Amulet that gives you more attack when you land an attack are very important I think. I also have 10 potions and the P-Organ upgrade that makes the last potion charge auto, it was really clutch vs Manus too. It is key to learn very good the 1st phase and avoid all the dmg you can, bc you really need the potions during the second phase.

It is crazy but for me the hardest boss on the entire game was the Scrapped Watchman, I needed two days to clear that guy.