Thank you very much for taking the time to leave this comment, very appreciated!
I'm taking the suggestions you leave me so further work goes more smoothly.
Regarding the leveling thing, I made the demo to be more of a casual thing so that's it, but after you clear the game there are some extra bosses plus a secret boss somewhere in the last level, for those that want to get a little more!
Oh, and regarding the credits yeah, I realized I didn't put a file with the credits on the ZIP, so I think I'll post it on the game's page!