after you got the boss key what you do is go to the bosses door. an easeier way to do this is by just saving and quiting then turn it back on then you should be at the begging of the stage then you take the little pot that makes you warp to a nother place (but stop as soon as you get to the yellow one if you oppened it ) then you look near the door and up a little bit and you should see a grappling thing use your rope coil and swing to the other side and there you go you are at the boss  if you need more help then just ask | 
