Only after 2 deaths I was basically impossible to kill me with got boring, maybe it would have gotten more challenging if I continued to play but the monsters were getting stronger to slow so I got bored before that.
I would also like some variety, maybe a more interesting map/different maps for progressing through the difficulties and a boss monster every few difficulties would be a lot of fun.
Not my favourite game right now, but with these additions I think it could be a fun game because the core gameplay is a good idea!
Also you should explain how to shoot and play the game because at first I was a bit lost on what I was supposed to do,