There are numerous Arenas within the main game mode that unlocks a playable minion character upon completion. The Arena has to be done in Single Player however, as multiplayer will just pit you against your buddies.
The earliest you can unlock this is to beat the King's Arena near the beginning of the map which unlocks the Barbarian.
Use this character to then defeat the stage Barbarian Boss, located directly after Barbarian War. Once you defeat the boss, trophy pops.