The Manic Finish wall is located in the Sunyshore City area, and it's a hidden treasure that you can find by using the Dowsing Machine in the right place. You'll need to find the specific spot where it's buried, and then use the Dowsing Machine to uncover it.
Here's how to find the Manic Finish wall:
1. Go to Sunyshore City.
2. Equip the Dowsing Machine.
3. Use the Dowsing Machine in the area around the Sunyshore City Gym, especially the area to the right of the Gym, near the water.
4. Follow the Dowsing Machine's signal. It will lead you to the buried treasure, which is the Manic Finish wall.
The Dowsing Machine is a helpful tool for finding hidden treasures throughout the game. Once you've found the Manic Finish wall, you can then equip it and use it in battles to boost your Pokémon's stats.