This has been much discussed on Discord - you may wish to search there for an answer, many have been given. The channel is pretty active with lots of knowledgeable and experienced players participating. They will be able to help you.
The summary of the discussion has been (including those that have submitted support tickets), that the passing this last building with 3 stars is pure luck - you have to just keep doing it until you have been successful. Some have been lucky within days, others not so lucky over months. Discussion there has equated this to a lottery ticket - chances are x in y ... every time. You have no better a chance the 101st time than you did the 1st. [Discord participants have had this confirmed by Support.]
Summary has also concluded that gear does not play a particularly large part in things - many have eventually been successful, both with, and without, any particular piece of gear.
One of the latest tidbits has been ... chose the path that gets you to the boss quickest / least other batles getting there. If you don't survive the boss with better than 50% remaining, bail and try again. Perhaps another day.
Many on Discord feel your pain. You may want to have a look / see there.