dude its easy, and he isn't behind a door. in the large room with the white light boss door and 2 giant knight dudes you go up the stairs, then run around, you'll see a broken window, run past it, there is an opening in the wall, go in it, there is the blacksmith
When I go up the stairs there are 2 large knights but they're not guarding any white light boss door. And the broken glass that I think you are talking about is incredibly far away and when I go through it there isn't any opening in the wall. And I watched a live streamer play not too long ago and that huge dude hammering at an anvil WAS behind a door.
you are deff in the wrong room. the room i am talking about is HUGE. the white light door, is HUGE. the knights are HUGE. they are atleast 10, 15 feet tall. about as big as the stone boss from sen's fortress. there is also one of those silver knights using a bow and arrow on the opposite side of the room from where you enter.the bow and arrow guy is by the opening in the wall where the black smith is.
so when you enter the room there is stairs to your left. you can go down some stairs to fight the big knights or go in the white light to fight the bosses, or you can go up some stairs. if you go up the stairs and follow the path you will go right by a broken window. you can go through that window to get the dragonbow or whatever its called, but you don't have to if you don't want to. if you go past the window, the path leads to the opening in the wall. its not far from the stairs where the guy with the bow is. its on the side of the room opposite the side you entered. you'll know you are in the right room because you will see the huge knight guys and the white light door