There don't seem to be any. I've been trying to figure out the 'hardest star' for hours!
The one with the sign that says "might be the hardest star in the game"? That one's not actually so bad, but I'll admit I kind of found the solution by accident - but I might have realized it was the solution if I'd remembered that room while I was playing the end-game.
Part of what makes this one hard is that it combines multiple game mechanics, some of which you don't really learn about until the end of the game. If you got the star in 9,14 already you know a number of those mechanics. The last mechanic you don't learn until the end of the game, but you find out about it in a giant room full of kill-flags.
I'm only stuck on two last stars - one I haven't even found yet, and that one in 14,17. I already know there's a lot that can be done in that room that isn't immediately obvious, but I haven't figure out any way to abuse what's in that room to get a cat into that little hole. I've tried pushing a dead cat up and then pushing a flag onto it, but the medusa stones it right away. And I can't see any way to get a cat facing the other direction there.
I can't believe this... I figured this out just a minute before I saw you'd posted a hint, because someone else mentioned what room to use. But I'm annoyed at myself it took so long because I've used every game mechanic this required repeatedly to solve other stars and secrets, and yet every time I looked at this one I just kept thinking "but, my cat will turn to stone, so I can't come in that way". /facepalm