There are many ways to improve your characters, besides simply leveling them up. You’ll often earn rings that you can equip on a character which can improve statistics. Go to the squad section and hit the “i” button next to the character that you want to equip, then “put a ring on it” (hahah… groan). Voila, higher statistics!
Of course, you can spend embers to improve your characters’ statistics. Go to the same “i” screen and hit the plus sign next to either rank, attributes, or ability. Rank in particular is the most important because this is the only way you will be able to reach higher experience levels.
Do research, too, on your “Rituals” until you unlock evolution, and you will be able to put two identical cards together in order to get one more powerful card. You can also get evolved cards through the usual methods (catching them and acquiring them from the shop). Any evolved card will have a “plus” sign next to it.
Need free diamonds? When the popup to “rate the game” comes up, tap it. It doesn’t matter if you actually post a rating or not, all that you have to do is to go to the App Store, then go back to the game. Next time that you reload the game (due to either closing and opening the app or due to a server error or a bad connection), you will earn 100 free diamonds.
Always look to a new hero’s initial stats when you are trying to figure out if they are worth bringing into your party, as well as their rarity (which is represented by the word in the upper left corner of the info screen, not the star rating). Then to get their experience levels caught up, go to the “fortify” area if you have researched Ritual 3. Or if not, go to old battles that you have already beaten (which will typically be easy at this point) and beat them again.
Want to collect embers immediately? Do the exact same time lapse trick that you do for the energy. Then go back to the game and collect from the ember baron. Repeat this until you fill up your embers, then use them and repeat it again, etc etc.