Hard: Do the hardest Puzzle map there is (Sweep this!) and survive for as long as you can until you get a score of 500. You don't need to finish the game to get the badge. :)
Could there possibly be a way to 'lock' columns and rows which you are certain are correct? There's nothing more frustrating than accidentally clicking (and thereby changing) a square you already know to be correct, then having to look for it, especially in the later levels T__T
What would you want in this practice mode ? I thought about it but haven't figured out a good way to implement it.