Hi!
At first I was confused what the diamonds do when you pick them up, but then I found out that you are using a rock-paper-scissors mechanic to make certain enemies weaker or stronger. I think that's a great idea but I would have sold it a little bit more by maybe changing the player sprite color too and playing a screen effect every time you transition. So it is more obvious what exactly happens when you collect those diamonds.
Maybe an upgrade system would be cool as well, where you can chosen an ability or whatever every time you complete a level. This ability could then also change with the form the player is in.
I was missing sound effects. This would have made you game so much more polished. There are a lot of great libraries online like freesound.org which can help you create nice sound effects.
I would also add a little bit of screenshake every time you hit an enemy or the player get's hit. Also some kind of bloody screen when the player gets hit, would make it more clear that you got hit.