As the description says, you need to get a champion up to level 15. Your sovereign does count as a champion, so just keep on sending them out into the wild to fight, explore, and quest. Multiple champions in an army share experience, so kick other champions out of the army if you want to expedite this achievement.
With one other champion in the army, and fighting/exploring as much as possible trying to find bosses for other achievements, I got this achievement very late in the game - with about 100 turns left before time ran out.