This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for Far Beyond Frieza side quest.
Time frame: From Majin Buu Reborn - Episode 2: In Hot Pursuit to Episode 2: In Hot Pursuit
Reward: 255,040 EXP, 3x D Medal, God Medal
Where to start: Great West Desert Area, as Gohan.
To start this mission, you must go to Great West Desert Area. Meet with Frieza - he will give you an additional story.
Your task is to defeat the opponent. Frieza will be at your level - you shouldn't have problems with defeating him. Win the fight to complete this substory.