Obtained: Speak with Nappa in the Central Plains Area after using the Dragon Balls to revive him
Objective: Defeat all enemies
In order to fight Nappa again you will need to collect all seven of the Dragon Balls and use your wish to bring him back. To locate him, head to the Central Plains Area and look along the eastern side of the map. Speak with him as Gohan to begin the battle (you can change player characters in the party menu). Defeat Nappa to complete the story.