Honestly, it looks like you downloaded some visual game creator and made this in 10 minutes. I have nothing against that, but the game isn't even enjoyable. Look:
1 - Too slow paced;
2 - Controls/dynamics suck;
3 - Unpredictable collision check. This means that you have to always target the exact top of the every enemy if want to guarantee that you won't take damage. With time, this can annoy/frustrate/stress the player. We play games to have fun, no to be stressed;
4 - The "take damage and get pulled back" things is totally annoying in ANY platformer; try the "temporary ghost" approach next time, or maybe the enemy explodes when it damages you, or maybe you could attack the enemy from distance. Do anything, but don't do this poor mechanic again. It rarely fits in a platformer and definitely doesn't fit this game.
5 - The challenges don't match the poor controls;
6 - The tunes simply don't match the game;
7 - Colliding to the sky?! Seriously? The top of the visible area isn't solid, unless you place something there to indicate that it is. I died many times trying to jump higher than the "screen". Colliding to the sky?! argh!
It looks like a first effort. Take this as constructive criticism.
Thanks for the effort. As you can see, if you simply tweak some variables in this game it can become much better.