SpongeBob SquarePants Underpants Slam!

I would recommend saving this for last as you will probably unlock it naturally anyways. Personally, I unlocked it on my third playthrough of Atlantis 2.

If you've beaten the quest mode with each character and the time trials and have managed to not obtain Clumsy! then load up any Flying Dutchman or Atlantis level and fall off the bottom of the screen.

This should be simple and self-explanatory! Play on a level that has an open bottom which you can fall from. Simply run to the bottom and jump off, and keep jumping off until this Achievement unlocks.