Thank you for the review, what you say about changing color of the enemies for variation is something I have in mind, the mastery tree I will try to make more understandable. The interaction of fireballs is intended, I want people using it as a shotgun be a valid strategy.
Making the crystal disappear after some time I think is a bad idea, I played many of these kind of games and destroying enemies while positioning on the screen order to get some crystals is part of the difficulty and challenge. I don't think people who like survivors-like games would enjoy it.
I will take notes on that topic of the enemy curve, is gonna take some time make a good one, specially because I wanted to be 20 minutes... but I am working on new enemies and bosses, so I will have enough variety in the future to fill those 20 minutes.
By the way, did you try the ice spikes or thunders? right click let shoot icespikes that slow and pierce enemies, thunders are a power up that appears when lvl up that make land lightning strikes at ramdom spots of the screen.