This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for Super Saibaman?! side quest.
Time frame: From Majin Buu Reborn - Episode 6: A Shred of Hope to Episode 6: A Shred of Hope
Reward: 233,453 EXP, D Medal, Improved Scouter
Where to start: Western Area, as Gohan.
To start this mission, you have to go to West City. There you will find a Saibaman - he will give you a side quest.
Your objective is to fight one of the most difficult battles out of all the side missions. It is best to take a team with you. Your goal is to defeat several opponents with levels slightly below yours.
After that, you will complete the mission.