Item description:

Not sure if anyone made it available yet, but here is my saves for mortal shell, first save slot is my main character on ng+2, not sure if you can unlock anything with it, and the second save slot is at the last boss needed for Forever Alone and Life is Suffering trophies.

I recommend backing up your own saved data before re-signing everything.

Load second save slot.
Go upstairs and enter the hole where the chained NPC that asks for Glands was to begin the boss fight.

This isn't the best strategy for the boss, here are a couple of tips that helped me and maybe will help you, there are probably videos on youtube with better strategies and explained a lot better than this.

Remove everything from Quick Items and place only Martyr's Shawl and Tattered Vestment from "unlimited use" tab in the inventory menu.

-Martyr's Sword is maxed out, use that for damage and Hallowed Sword for summons since its quicker(they get one shot anyway).

-At the start of the fight he will charge at you with an arm swipe, lock-on, while standing still press dodge button for a backstep, has enough i-frames to keep you safe and you can start spamming light attack on his face right away, 2 to 4 attacks on his head should be enough to stun him, backstep to avoid getting hit by his head and spam light attack, keep attacking even after he gets up, if he starts an attack animation after getting himself up, harden and try to hit his head for a second stun if possible, if he jumps back and charges at you the same way, repeat the process, when waves starts to come at you, run towards it and roll dodge, harden after rolling the second wave if he's already charging at you, if you have enough time before he charges, backstep and repeat the process.

-Hardest part is dealing with the summons during the second phase, i switched to the hallowed sword by using Tattered Vestment in the quick items, dodge their attacks and light attack after, if you're about to trade hits, use harden, switch back to Martyr's Sword when going for the boss again by using Martyr's Shawl in the quick itens menu.

-Trophies pop after killing it and ascending, if you decide to not ascend, you'll be teleported back to the hub area and you're gonna have to kill the boss all over again.

That's it for this boss i guess, he has different attacks but since dodge in this game has a lot of i-frames and it's op, dodge or harden should be enough to avoid any attack animation.