There is absolutely no need to pay the valuable coins for unlocking them, although that is a possibility. Instead, just play the game and when you reach a specific stage and beat it, you will automatically unlock that hero for free. This is the way to go! When it comes to leveling them up, the only way to do so appears to be by Spending Diamonds. For this reason, it would be best to find the hero you enjoy playing with the most and focus on leveling that one up. I personally found that the starting hero is really good because of its energy skill that removes curses. All the others deal some sort of damage which is useful at taking out monsters, but sometimes those curses can be a real game killer.