Forum:Obtaining Hero Medals

I've searched around the CA Wiki trying to figure out how to upgrade my heroes beyond level 4. I found information indicating that some heroes (though suppossedly not all, such as Zin) need hero medals to upgrade them. Aside from up to two hero medals for Sophia, Strider, Penelope and Dragan through Celestia's Quests, how do I get other hero medals? For example, I have the hero Mephistopheles; how do I get him higher than level 4?

Thanks for your help in advance. :D