After a few runs I really ahd to point oit last boss. (Sorry I don't post that much on forums but kinda weird no one has pointed it out)
1. Regression/thrall bug; this is a problem that lingers since NM.. after regressing the boss you can end up in the pool and die. (A way to avoid is to not use movement keys as you use regression...)
2. The Shield-Sword-swordcross-shieldbash-donut (idk how to call it xD); I usually end up dying even though I frame into the boss after his first wave has been cast. I tried not to use movement keys as well but it didn't work and this is very frustrating as a healer because it's usually a one shot for me.
3. Circles outside soul world. SOmetimes they just kill you even though you have enough hp. I even cast Kaia but still ends up killing me sometimes
4. It only occured once so I'm not sure why, after he leaves the soul world here you have to sleep him can bug out. I slept him and he was slept for a second but still ended up wiping...
I think that's all I have seen during my 4 runs for now.. If anyoen has something to add, feel free to!