This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for the Toughening Up side quest.
Time frame: From Majin Buu Reborn - Episode 1: Good News to Episode 2: Time for the Tournament!
Reward: 243,635 EXP, 10x D Medal, Ultra Sacred Text of Martial Arts
Where to start: Western Area, as Gohan.
To start this mission, you have to go to West City. Talk to Trunks - he has a substory for you.
The goal of this mission is to defeat the quest giver. He shouldn't be a challenge for you. Defeat the enemy to complete the mission.