This level is a fog level walking through the forest. As you spend time not capturing Pokémon, the fog will deepen until it is so thick that you cannot progress further. If you manage to collect a decent amount of points, by the shrine, Celebi will appear. celebi isn't a boss but does have a unique battle method. You need to type its name multiple times as it'll rewrite time whenever it's close to capture

Bronze Medal Requirements:
Clear the area of fog and investigate hte mysterious shrine. Capture Celebi at the shrine

Silver Medal Requirements:
Get a no-miss capture streak of 10 and capture Celebi

Gold Medal Requirements:
Earn 250,000 points and capture Celebi