This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for the Master side quest.
Time frame: From Android Terror Arrives - Episode 8: Fleeting Normality to Episode 8: Kami's Successor
Reward: 116,001 EXP, 5x D Medal, Card 039, Select Beast Meat, Frozen Rabbit Meat, Fatty Giant Fish
Where to start: Southeast Islands Area, as Goku.
To get the task, you must go to Kame House. Speak with Master Roshi.
Your task is to defeat Krillin. You won't have any difficulties with this. Your opponent will probably be several levels below your character.
Win the fight to complete this mission.