This level is a complex level. In the Japanese version, you'll come up to various crossing points where you have to select the right way to go. Take the first two Pokémon on the right when given an option, then the next left and then the next right. Get to the end and you'll find Shaymin. If you hit all the flowers, it'll transform.
In the English version, the line is a straight line with no junctions, but a time limit. You have to reach the end in 60 seconds, having to capture Pokémon to make yourselves go faster

Bronze Medal Requirements:
Get to the end of the level

Silver Medal Requirements:
Capture a Sky Forme Shaymin

Gold Medal Requirements:
Capture a Sky Forme Shaymin while having a Pokémon streak of 10 or more