In the game BattleJack 21 you yourself have a level and then have cards who have independent levels. You level up yourself by gaining experience. You can gain experience in several different ways the main way is by completing Dungeons and progressing through the story. Each level of the dungeon will provide a certain amount of experience depending on the difficulty and mode (normal, nightmare, hell).
Other additional ways of gaining experience are by defeating the Chaos Titan (350exp each defeat); Winning a duel in the Arena; and completing Quests.