Very nice concept! I found myself following a strategy of just following the crystal and shielding every once in a while, as the enemies bullets would usually miss the crystal, especially in larger arenas. This made it so I didn't really have to worry about my health or the other enemies all that much. Might be something to look into. Other than that though, the graphics and music were great, the concept has a lot of potential for strategy and expansion, and it's very well made. Great work!