A great level that, while it doesn't only go from left to right, is still linear, which suits us perfectly! The level is hard but that's justified since it's relatively late in the game (except for a couple of jumps which I'll address later). First of all something needs to be done to the Shadow Realm background as it barely differs from the Light Realm one. Also, the skulls are a nice touch but they stand out too much and can be mistaken for real Skull Rafts sometimes. Darkening them a little should fix it. I can take care of the background. Another minor detail is the door to the Shadow Realm. Keep in mind we'll be using a custom door throughout the episode so either use the Magic Potion, or a SMB2 door if you don't want to mess with the Potion. I added the globally used graphic to your door structure as I thought it was cool. I just thought I'd point this out so I don't have to do this for every submission.
Finally I'd like to address two particularly difficult jumps. These should be nerfed mainly because making it on your first playthrough is out of question as there's no way you can know you need a well-prepared running jump or you'll fall to your doom. I know it's hard mode but we're not going for unfair challenges.
Overall it's a fantastic level, though. Great job!