So basically:
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A hard but short section on the top.
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An easy but long section on the bottom.
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A difficulty in between and an average length in the middle.
First, start with the easy but long on the bottom. Have jumps that are way easier to make. Put large props and barriers to make it easy. Use the change size button to change the sizes of props/barriers! For the middle section, place slightly smaller things to land on. In this section, if you fall, you are knocked out. Use zones to knock out players in the “bad areas”. Stop short of the end of the easy section to make it shorter, and the player can walk the rest without having to worry about getting knocked out. Create a short hard section, but have super duper po hard jumps. Have the hard section stop before the medium section stops. Use the following guide to help with jumps.
oh- almost forgot! Here are some search results for sonic.