This is the best implementation of sharing a health bar with your enemies that I've seen so far. It forces this very defensive style of play and you actually give the player other, non-violent ways to defend themselves so it doesn't just become a bullet-hell. Great job!
I did notice though that the shades all target me - not the crystal. I realized then that I could win by just running away far from the crystal so it would never get shot at, and then I reverted to kinda bullet-hell-y tactics. I didn't feel like I was defending the crystal - I was just defending myself. Having the shades target the crystal, even if it's not constantly, would help me feel more like I'm defending it instead of just defending myself.