First off: The game is really fun, I really like seeing adult games with actual, fun gameplay, well done! Now, I noticed some of the fights are really, REALLY tough. Like for example earths later stage seems almost impossible, or Mars. Could be a skill issue of course, but still wanted to point that out. Also thought maybe I am missing something? 

Thank you for playing! I'm glad you found the game fun.
If there isn't a bug at play, everyone is possible to defeat.
There are some things that I don't explicitly say or may not be too intuitive:

Before facing the Sun the game is non linear, but this is the intended route:
Ceres (Initial area) --> (Go down to the dwarf planets zone) --> Makemake --> Haumea --> Eris --> Pluto --> (Go back bottom left to the major planets zone) --> Mercury --> Mars --> Venus --> Earth --> (initial area, dwarf planets zone again, top right) --> Neptune --> Uranus --> Saturn --> Jupiter --> Sun --> Black Hole.
The positions of everyone are a bit weird because I tried to mimic the real life positions of the planets in the solar system.

Important: You don't level up by defeating bosses, you do by defeating regular enemies; they change color depending on your level, if they are white you are good to go, the redder they are the harder the fight will be.

The speed of the characters is also based on the irl planets. This influences formation: the smaller ones like ceres should be on the front since they are fast, the bigger ones like jupiter should be on the back (slow while focusing is better).