OK.. nice so far
This game is far from done. It's an aim and shooter, but you've only provided one enemy.
Also, said enemy has a chance to shoot you while flying on-screen (before they can be shot to prevent damage) this means you will eventually die regardless of your skill.
There is no way to regain your shields. I would suggest a HALO style shield display, wherein your shields recharge after not being shot for a set amount of time.
Add Photon torpedoes.
Add other obstacles, like meteors etc.
Add a shield bubble effect when you're hit.
Other than that, great job. The phaser following the enemy on a direct hit was a nice effect.
Having a targetting blip around the enemy when you've got them sighted would be a nice touch.
Good luck!