There is currently a bug that registers the combined levels of all heroes in your "deck", all will gain exp even if they do not participate in the battle.
Sighris said:To add all 4 heroes, 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've done this, just join some games and battle until your hit level 7 (level 7 x 4 heroes = cumulative level of 28), and this will unlock.
This works for the following achievements as well.
(reach level 2 with a full deck)
(reach level 4 with a full deck)
(reach level 13 with a full deck)
Thanks to Sighris for clarifying how to get the other two heroes.