This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for Perfecting Perfection side quest.
Time frame: From Majin Buu Reborn - Episode 8: Finale to No time limit
Reward: 28,319 EXP, D Medal, Training Medal, Card 066
Where to start: Center of the continent, as Goku.
This mission is only about fighting Cell. He won't be a threat to you. However, you can take a team with you to finish the fight faster.