Formless One Bug

Guys, this is happening because you are doing way too much damage too quickly. What you need to do is (disable the cheats) '' slowly '' but surely whittle down his HP. I'm also pretty sure you need to hit his legs when they light in this purple glow. Eventually, he flies up, and does a round of his electric breaths like at the beginning when the fight starts, and then comes back down. Afterwards, you start hitting him, and he eventually lays down when you've hit enough and is at a weakened state and starts taking a lot more damage from you. Then after this he goes to his perch and the fight continues normally as it should, instead of staying there with his HP bar not moving after you've reached the '' limit. '' 

So there seems to be some checkmarks/events you need to hit in the fight before the perch section, otherwise the game gets confused and you get stuck in that section.

The problem is third party program, if u still wanna use it,
Use it when u fight and stop it when he fly to the place its get stuck,
when u will do so its will let u continue the fight (u do it twice between stages

I did enjoy the fight without any third party but it is indeed hard fight.

Having the same issue, no matter how many times reload. Just decided to start a new game. Which sucks, cause was 30 hours in.

A empresa não fala sobre sobre update ? Bug dos grandes que raiva....

I can confirm this, ty HausOfNadav...

 I used WeMod and I had to pause the game everytime the formless one went to the cliffs, click the "x" to stop WeMod's cheats from being enabled, then go back into the game, unpause, dodge everything I could and within about 5-10 seconds The formless one proceeded to call henchmen (Undead) and come back down again to fight in the arena. I then paused again, re-enabled the cheats an fought him until he went back to the cliffs. I repeated this process two times in total to kill him.