Every time you complete a successful raid, EXP is gained. This EXP is split between all the heroes you put on the battlefield. Use strategies to ensure a three flame victory in order to gain more EXP at a time. Place your best or your worst heroes on the battlefield, depending on if you want to make your best heroes even better, or get your worst heroes started on becoming stronger.
The higher level the person is that you attack, the more EXP you will gain. Attack bases that are high level in might, but have low enough heroes and buildings for your heroes to defeat. This will also make it a sure chance that you will win three flames, and prevent your heroes from dying so that you don't have to waste gems or time trying to revive them.