Get your medals from fair and labyrinth. Ive gotten them for as cheap as 2 or 3 gems in fair, and free on the card flips in labyrinth treasure chest squares. Stars are just one of many methods to gain power in game also... try and keep that in mind. Personally I do not like the fail rate but I mean there are levels that do not lose a star when you try and upgrade and fail. I've failed 3x today trying to take a purple hero to lvl 5, and haven't lost a single star. Just didn't gain one.