If you end up getting stuck and unable to progress, set the game on auto battle and play and replay previous stages. You will earn all sorts of rewards from there, loot that will allow you to increase your heroes' levels and make them better and stronger. The game is actually built nicely and you will rarely have to replay a stage in order to progress: completed stages have increased difficulty ratings available after completing them on the previous difficulty setting, so make sure that you check out all options before deciding that there's no new battle that you can take on.