Honestly I think every part of the game is definitely doable except for the safari zone, which will probably never be completed. Other than that I can see the Celadon Team Rocket boost puzzles being very difficult, as well as the rock pushing puzzles in Victory Road.
There's also another ledge on the way in to Victory road with the extra wrinkle that you need to go down *just* far enough and not too much before you can start going on it.