Interesting way of working with the "Only One Bullet" !
Like other people said aiming is very hard and enemies spawn way too fast for someone to get along with the controls and aim. Music wise put the volume a bit lower :).
And for the text I see you used the basic text from Unity. Don't. Like never. It sucks for real. Use TextMeshPro, it's free and available directly in Unity. You might just need to import it. But you'll definitely see the difference ;).