I'm sorry if i'm wrong, but i'm here sharing thoughts. For my experience, its based on the player's levels and decks when your playing online, for example, if you have a level 25 Squire and you have a friend that has a level 50 Monk, then the mobs will be around maybe level 42-50. So really, i recommend saving the defender medals on the other heroes you find the coolest. there is also a mysterious portal and the end middle of the public tavern, where you can play and test the heroes that you don't have yet. Hopes this helps! :D