The next level is the first level which contains a boss. In the forest, you will encounter a variety of Pokémon.

Bronze Medal Requirements:
Investigate the level in search of the Pokémon; Serperior

Silver Medal Requirements:
Capture Serperior at Level 70

Gold Medal Requirements:
Capture Serperior at Level 100

The boss of this level is Serperior. As it arrives, it will send out attacks on a variety of leaves in order to damage you. Take too long and it will leave. Once you've defeated the leaves, it's able to be captured.

Level 50
Serperior unleashes two leaves at any one time

Level 70
Serperior unleashes two waves including one set of vines that are continuously growing

Level 100
Serperior unleashes three waves including one set of vines that are continuously growing