These kinds of games always give me a headache. I like the Sudoku inspiration here but it's a little daunting having over 500 levels. Judging by the medals and what I've seen from the puzzles, beating all of them is not terribly important; they just add variety and play time to this game. It would be totally possible for this game to choose levels randomly - this would also eliminate the problem with players just guessing and redoing levels.
The exp and money system is also stupid. This is a puzzle game, I don't want to manage currency that barely does anything except change the game's visuals.
One thing I considered that would have been really cool is if there was a grid of several 10x10 puzzles arranged to make a larger, less pixelated puzzle. You could solve a big pixelated part, then solve individual sections in-depth to reveal a very clear image. You could also probably find a way to do this with colors; there's no reason to stop with half-baked pixel art.
Overall fun but way too long. Plus, players that are really good should be able to delve into Normal and Hard difficulty levels sooner.