Another page of the FF7 Remake guide contains a walkthrough of the Dynamite Body side mission in Chapter 9 - The Town that Never Sleeps.
Go talk to Sam and tell him about your accomplishments. You'll be dispatched to go to the Coliseum. Take the elevator down, heal and choose a new Match - Special Match. You will join the duel alone, without Aerith so you can wear Healing Matter just in case.
You must defeat the enemies - two Bombs. Opponents are not particularly strong, you only have to look out for their fiery attacks. You may also be tempted to steal their self-destruct technique with the help of Enemy skill matter.
After the fight, talk to Sam and you get the job done.
Reward: Arcane Scepter-a weapon for Aerith.