The level design is good, but the difficulty of the game does not result from the level designs being hard to navigate: it is because the left and right movement controls are very stiff and the character does not interact with the environment like he should. What I mean by the last point can be shown in level 9. When I try to jump and go through the small hole, the character refuses to do so unless his falling and upward velocity are almost at 0 when you are lined up with the hole, which is very difficult. Even when I made it through on one occasion, I couldn't get pass the spikes because you can't control the height of your double jump (looking back, you need expert timing to get pas the spikes at the ending). The fact that I can touch sideway spikes on their sides and still die is also annoying when they could have been used as platforms.
A difficult game is not just when it is hard to beat: it has to be when the failure of a player to beat it is due to a lack of their skill, not in the limited coding of a game making program like Stencyl. Still, I don't think you have to worry about this game passing judgment. I've scene a lot of bad stuff get passed into the game portal that this should be there in no time at all! Good luck on your future projects.