You should always use the lesser Elunes to upgrade the better ones, firstly the ones that you use the most. That way you will always be prepared to take on tougher opponents and you won't have to worry about pretty much anything! To upgrade your heroes, tap on the hero you want to upgrade and go to the Upgrade tab. There, you can either auto select or select manually who you want to "feed" to your hero to upgrade it. I suggest upgrading only the most powerful and best Elunes you got, because they will need a lot of upgrading and you will fall off quickly if you start upgrading every hero that you have. It's better to focus on a few good ones and use them all throughout the game.