For a first game, it is a nice little one.
Thing i would improve:
-Sounds! Have seen plenty of games that have some kind of shooting without sound, and it really needs something, so you receive some feedback. Also a sound when you hit an enemy with it and when you lose hp.
-The first boss threw a thunder at me just as he respawned, and it made it unavoidable. Make a cooldown at first so it doesnt shoot directly after spawning.
-Better Menu (Make it all in one scene with animations
This are easy things to correct, but they will have a huge impact on your game's quality.
Good job overall!