Go through the sewers. there is a stairway that you have to take. you will have to fight them to be the gang "Leader".
It's only accessible in Spring/Summer when the sewers are flooded. Requires Surf. (And probably Repels unless you want to fight a legion of Zubats.)
- Enter sewers and go east until you see a stairway(note, that is NOT the one)
- Go south. after you pass a janitor, there will be a wooden bridge thing right there. dross it. Follow that path til it takes you to the next wooden bridge.
- Now, there will be this area where there are three bridges leading in different directions. take the bridge that leads you East.
- Right after you cross that wooden little bridge,there will be a another one. Don't cross it, use surf and follow the path until you see an opening to the sidewalk.
- Follow the sidewalk south and after passing a door looking space, a little forward will be a space that leads you south.
- Go up the stairs and prepare to fight 3 people, one at a time.
Good luck with the medal! :)