In level 5, as you walk up the stairs(before you go down again), at the top, there will be a light to the right. Open it, and there will be a flemoid and a yellow key. After you get rid of the Flemoid, and get the key, turn around and go to the northwest corner from where the key was. It's a door. Inside, the will be a stream. Go right. At the end is a waterfall. Go down it. There will be another flemoid in the room you are in. After you go out of the room, you are in the boss's room.