If you want to find a hard demon level in Geometry Dash series, try the one named Geometry Dash Dance Massacre and you will not disappointed. Believe in yourself and focus on winning, this level was designed with high speed and many traps on both ground and air so that players have to be flexible to get through. Fly through the giant cube with moving spike at the middle like a real hero makes effort to win a monster. Or transform your cube into a thorn ball rolling to the finish line. Apart from those details, the zig-zag tunnel will be a signature of this difficulty because not everyone can pass through it and stay alive.
If you want to take another one in the list, I recommend you to access hard level and consider some great choices.
New version comes in the series and you can be competent in no time because it has the same rules with the others
Jump to overcome obstacles
Fly through the tunnel
Go through the portals to change your look immediately
There are so many attempts you can try in this hard version