I had trouble with The Plain stage as well. It took me forever to realize that I was missing a trophy from that stage. Remember at the beginning (before the movable wall) there is a bunch of blocks stack on top of each other in bout 5 columns? Well cut your way through the blocks and try to stand on top of them and cut away at the blocks that are somewhat hidden at the top of the screen. A trophy dropped from there eventually and I'm thinking that's what got me the crown completion.
Hope that helps. GL