The Ultimate Gym Leader Challenge is waiting for you in Sunyshore City. When you arrive, a member of the Elite Four greets you, asking you to find the City Gym Leader, Flint. He didn't have any worthy challenges and so he got bored. He's not in his gym, so you have to bring him back to earn the last Gym Badge. In this guide, we will detail how to find Flint Gym Leader in Pokémon Brilliant Diamond and Brilliant Pearl.
When in Sunyshore City, we recommend that you go up the bridge path that overlooks the city. The entrance to this path is near Route 222, where you initially entered the area. Follow the path, head south and then east. Next, you want to reach the City View Lighthouse. When you enter, you should see Flint looking out of a periscope.
Talk to him and tell him that you are in town to challenge him to a Pokémon battle. Then, with renewed vigor, he will return to his gym and prepare to fight you. Flint is an Electric-type Pokémon Gym Leader, so it is highly recommended that you use Rock-type and Ground-type Pokémon against him when facing him in battle.