Challenging levels aren't Kaizo, you don't need to have Paratroopas needed to be jumped off, Foos blocking the entire path, enemy walls, a crapton of enemies, or something like that. For a GOOD and yet CHALLENGING level, try to learn how to use events like Tile Gods or Locations. With those you can make challenging puzzles. Watch Martboo's custom levels on YouTube for an example of a challenging yet good level.
_________________________
Hello. I'm boring.
I do boring things like being boring and making everyone bored.
I say boring things that don't make sense. Whatever junk is above this signature is most likely boring.

Games you should try...?:
The Binding of Isaac: Afterbirth
Shovel Knight
Terraria
Undertale
Touhou
Don't Starve: Shipwrecked