Obtained: Speak with King Cold at the Kodaiho Lands in the Continent Center Area during post game as Gohan. You will need to use the Dragon Balls to revive him.
Objective: Train with King Cold
Head to the Kodaiho Lands and make your way to the northern part of the map to find King Cold. Speak with him to request a sparring match. King Cold will send you to defeat three groups of his minions around the area before he accepts your challenge. Defeating them triggers a cutscene completing the sub story.