Huh, I never knew about the Whomp King thing before; I'll have to check that out.  And I concur about the hardest Green Star being Cyclone Stone 1; originally I thought you had to jump from the oddly sloped part, not the ring with the Tox Boxes that it's situated directly above.  It was on some subsequent playthrough I figured out you could get it from the ring, which made it substantially easier.  Still by far the hardest on a first playthrough, at the very least; how you're actually supposed to collect it is very unintuitive.