I haven't managed to get in to the previous room or in the bridge to get any checkpoints and perhaps meeting a criteria for the boss to actually appear since there is some something we must do or else he just wont appear entering from the left side does not seem possible but there may be some other holes on the right (some way to see the invisible walls and triggers would be amazing for this lol)

Update:

this requires you to light the lighthouse and go through pretty much all of clotted avenue to get the third metal box/barrel (unless I'm missing an earlier one or you can duplicate one which I think I managed to do somehow when playing casually) making it pretty slow on top of that you don't get the green crystal and you are not very well equipped for the boss (maybe some upgrade destroys him idk)