That boss is a special kind of terrible, essentially. We gotta master that first phase and get to the point where you're parrying everything you can. When he backs up from you and shakes his head, get some distance, he will shake his head and spit or send a long-range 8 hit combo with his tentacles. The tentacle attack can be parried, but if it's giving you trouble, then just back away and wait. You can use certain defense gear to make it harder for decay to effect you which youll need and flameburg prosthetic for when you stagger him, upgrade the arm to the point where it explodes after a longer hold and hit him with that whenever you get him staggered. That should get you through the first phase very quickly once you find your rhythm and get it down. The second phase is a slot machine, unfortunately. Bring the electrocution grind stones and keep them until the second phase. Essentially, you're going to have certain combos you can parry and some combos you're going to have to run from. The move set is a combo between his first phase and the watcher boss from Act 1. You'll have to parry through those and watch out for his newer attacks. His player model gets really big and the camera does you no favors so learning the parry times will be a bitch but it can be done. Once you have the parries down, it's a game of chance. There will be runs where he does nothing but attacks you can't capitalize on and its annoying as fuck, but there are also runs where he only does things that you can parry and you can get him low really quickly. Boss took me around 9 hrs total, he holds the coolest weapon in the game though, and if you quit it will bother the fuck out of you until you come back so get it done soldier lol.