I really appreciate the idea, it was easy to grasp and fun to play! I'm afraid I'll to have to agree with other posters before me in saying that the difficulty is quite high: I'm not very good with shooters so thi was particularly tough for me!
As a suggestion, in case you wish to continue development, you make more diverse enemies, some of them with lower abilities (for example lower firing rate or less powerful enemies). If you show up only the "weak" enemies first, you get a more gentle learning curve and a more enjoyable and diverse experience! Win Win!