Flying At Tree Level In The Harry Potter GOF PSP Game, How Do You Complete The Level Where You Are Flying Over The Water?

In the Harry Potter GOF PSP game, how do you complete the level where you are flying over the water? - flying at tree level

This is the level where you avoid the trees, must fly near the end of the level, over a lake, but I've always bronze or silver. Is there an easy way to finish this level?