You do realize you don't have to fight in the big trap room until the post game right? Because the only thing of note in there is the access point to the locked gate area.
If you end up on that floor, just hit up the menu and return to the entrance. Progress is saved regardless and there's no penalty for doing it. Getting to the boss and the weapon is entirely unrelated, because if you end up down there you must have overshot the entrance to the place you're looking for.
I am post game. I was level 60 so I was able to do it. I finished Costlemark last night lol.
Actually that gauntlet is if you take the wrong stone or choose the wrong path.
