I was able to beat the game on my second attempt. It was a bit unintuitive that the lives went down after the invulnerability timer, but otherwise the game was straightforward to play. It was a good idea to use left click to start the game, so you know it's an input, but adding the controls to the description would also be helpful!
Yeah, I thought the same as I was releasing the game the invulnerability timer was mostly only a fix that the bullets won't hit again but I could have solved it by just despawning it whenever it hits the player making it much harder when he has the shotgun as the active weapon. There is also a bug that is when the enemy moves too far to the left so that he falls down. But thanks for the feedback !