Not too easy, but not too hard. I managed to beat the boss on my first try without taking any noticable damage; of course, the lesser enemies managed to whittle me down to a mere fraction of my health, so I'm not complaining. The difficulty level is perfect, the coordination with the music is awesome (it reminds me of another flash that revolved around that song), and the entire package is overall great. I'm only giving this a 4/5, however, because there's little to no replay value. A high score table may fix that, though.