Kinda stuck on the Bahemut boss

He has the regen node and I get him to 40% but then he gains health so everything I break his shield , he just goes back down to 40%. Am I missing something? In the node it says he can't regen up to the previous stage..... yet he still does
0
Comments
edit: sorry, I read again and missed that he was going to above threshold
Switch off ur Willpower mastery or use Apocalypse