Bolgin Serilis not killable due to insta death at high levels High Keep The Pickclaw Depths Heroic He puts a curse on you that insta kills you if not cured. Clicking on his pot of Brew either before or after he casts the curse does not clear the curse. The Pot also does not become clickable fast enough to even cure a 2nd curse when you are trying to finish the dungeon at high levels. He becomes immune to damage. He curses you. You click the pot. Nothing happens. You die.
Maybe you are killing him too fast? Slow down and pace the DPS? Also, in the Deity window, you can (if you are good or neutral) buy Mithaniel Marr's miracle that cures curses. You will only get one shot at it in a fight, usually, but it can help.
The main point is the Pot in his room that is supposed to clear the curse does not clear the curse. This was reported in a older thread in 2016 and still not fixed.
Next update, Bolgin's Brew will last the full 12 seconds instead of being removed after being damaged once. Also, the cauldron will always be clickable for the buff as long as the buff isn't active on your character.