For fixing the difficulty, I'd recommend starting with a certain amount of power and having it decrease slowly over time, and quickly when you sprint. This forces the player to complete the level within a certain time frame and be even more careful with sprinting, rather than being able to drift around and gather as much power as they want.
There's also a big problem where if the purple enemies dash off screen you can't finish the level, so you have to start the whole game over. It happens more and more in later levels.