I quite like the game. A few pointers. Having every enemy be killed in one hit makes the game play quite dull. You can press space and survive until the second to last wave. The difficulty is not the problem, but the simplicity in 'solving' the gameplay. One way could be to increase both your health and the enemy ship health. Then you have to react to your enemies, you are forced to doge and weave. These kinds of things is what make a game like this more fun.
The final boss is a good example. Its pattern forces you to move carefully if you want to hit it, but allows for safer play, but you win by reaching the end instead of destroying the boss. More interesting without per se more difficult. Unfortunately the game did begin to lag during the boss (do you make sure the bullets are destroyed when off screen so they don't take any more resources?)
Overall, a well made game. Good luck with any future project.