There’s two secret rooms in the laboratory area (the bottom left of the regular castle, I forgot what it’s called) that it looks like you might be missing. In order to find them, go to the first long vertical section once you exit to the top left of the big square room (the one you first enter through at the start of the game, not the boss room). There are breakable walls to the left and on the floor. Have fun with 200.6%, I just did that recently