A Dynamite Body is a Side Quest in Final Fantasy VII Remake that you can complete in Chapter 9 during the Wall Market part of the game.
Sam has a request for you and wants you to meet him at the Underground Coliseum.
Head down to the dungeon and speak to the Gatekeeper, where he'll give you the opportunity to select Special Match.
You'll then have to take on a fight against two Bomb monsters, which if you've ever played a Final Fantasy game in your life, will be familiar to you.
You should focus on one Bomb at a time. They can use their Inflame ability to deal Fire damage to you, as well as their Fireball, so be prepared to heal.
They are weak to Ice attacks, so throw in a few Blizzards, making sure not to get interrupted by the Bomb attacks. Once one of them is down, the fight is pretty easy to control and complete.
After the fight, Sam shows up and gives you your rewards.
- Arcane Scepter