This level is a fairly small level where you need to capture all the Pokémon that you encounter in order to gain access to the key. If you don't, Gengar will send you back to the beginning

Bronze Medal Requirements:
Get to the end of the level, getting the key, and capture Gengar

Silver Medal Requirements:
Get a no-miss capture streak of 6, and capture Gengar

Gold Medal Requirements:
Get a no-miss capture streak of 13, and capture Gengar