Thanks for the review.
Yeah the game was released kinda buggy. A friend of mine helped me to discover some of them and I have released an updated version of the game now.
Once you download the update you shouldn't become invulnerable anymore. The waves should also start from 1 now too.
The four legged enemies are stationary and spawn smaller enemies if not killed quickly. I hope I didn't explain the game mechanic too poorly: basically shooting enemies increases the bar on the bottom left screen. Once the bar is full you hear an audio and your fire-rate is temporary increased. And yup, you can simply hold the left mouse button to shoot. No reloading required.
Anyways, thanks for trying out my game :)