Making levels was easy at levels x-1 to x-4, but then it was getting harder as no ideas came to my head. Also the guy that made sprites have made some of the levels, but I made them harder.
My plan for making levels was like that:
1 - Make an already completed level with complicated path;
2 - Make it harder and harder by adding changing something or making some actions forced;
3 - Maybe add some objects which are not needed for the game (you can place them on the level so that they are not used)
4 - Replace some objects so the level is not already completed