This is currently glitched in our favor.
It is adding all heroes in your deck towards this and since your deck contains 4 active heroes 13 is the magic level. (4x13 = level 52).
Before you start doing the campaign, go to the tavern and hit
, This will open your quest log. There are 2 quests you can do in sequence that will give you enough medals to unlock character 3 and 4, the monk and huntress.
Once you start the campaign, you'll unlock this during the 4th mission.
It doesn't matter which character is active, they all receive the experience as long as they are in your deck during mission.