Answer:
Go through Corneria normally until you reach the checkpoint Falco will be attacked defeat his enemies with a bomb keep going until you reach the water drive your Arwing through the green arcs until falco says smooth flying keep going through the arcs until he tells you to follow him you will be sent through a waterfall continue until you reach a boss fly downward then start shooting at the guns then at the body to defeat the boss. MISSION ACCOMPLISHED! Now you can reach Sector Y.