If you thought the light world levels in this game were too hard, then you're in for a rude awakening in the form of the dark world stages, which ramp up the difficulty of every level by a huge margin. This does not include boss levels or warp zones.
Flipside is absolutely terrible. Yes, it's one of the hardest levels in this game full of hard levels, but the challenge here is not fun at all. Still an amazing game though, don't get it twisted.