I like the difficulty, it's perfect. I mean, I've lost several times, but at the same time I was learning, and it gave me satisfaction to pass each level.
It has the exact level of difficulty, for my taste.
Honestly, the boss part was the part that cost me and at a certain point made it not so much fun. When you hit the boss from behind, then he stands screaming and allows you to hit him. I loved it. The problem, (it's a super personal opinion, not professional), is that when I did that for the fifth time, I felt that it was a very repetitive "game loop", and that it would never end. I would have felt better if it were three or four times that this loop occurred, or at least know (by a life bar or something like that), how much longer I should hit the boss.