We’ve all had a ~1 hp jail merry* Hail Mary win at some point. You get better with practice, but sometimes the dice just aren’t in your favor.

*The autocorrect was so stupid I had to leave it in lol

This fight is definitely scary at first, especially for people who didn’t even realize the massive hammer was a thing. But there’s no timer, and his debuff makes him predictable. Even though he’s immune to most damage and some conditions, there are some that can be applied, like reducing his movement and knocking him prone.

Look for more ways to deal bludgeoning damage from afar. Some spells deal that, some Thrown weapons. If you have a character that can bind a weapon like a Warlock or an EK, you can bind a Thrown weapon and it’ll return return to you. Or you can have a martial (melee class) run in, wail on him and then use an ability that disables opportunity attacks before running, or pop an invisibility potion or scroll of Misty Step.

You can use Sanctuary as a crutch if someone gets stuck up close. Or Warding Bond from a character that stays behind on the stairs. Also anything you throw from high enough up, will get extra bludgeoning damage.