I was about to come here and talk about how bad your game is, but then I've actually read the description and found that if I press F I can resize the screen and my biggest complaint disappeared.
There are still some problems: sometimes the enemy will hit you only once and deal 50 damage for some reason (you remembered to give the player a second of invulnerability when hit, right?). Also, why is there such a long wait until the furnace reappears? I understand it was needed so that the theme can come in play but I think it would have been better and more fun if you made more enemies attack you at once with you one hit killing them all. This way you could have shorten that length to 10 seconds but still have a challenge and run out of space from killing too many enemies.
I of course like that you, like, made game and hope you a lot of luck with whatever you next project is.