Instructions
Walk to the third floor of the Canalave City Library after you earn the Canalave City Badge by successfully battling Byron, the gym leader. Talk to Professor Rowan and he will tell you about a problem happening at the lakes in the area and will ask you to investigate.
Fly to Veilstone City and travel west to Lake Valor. The lake has been completely dried. Battle with the two Team Galactic Grunts using your strongest Pokemon, then head over to the large cave in the middle of the area.
Enter the cave and confront the Team Galactic boss, Commander Saturn. Use your highest level Pokemon to defeat Commander Saturn's Golbat, Toxicroak and Bronzor.
Fly to Twinleaf Town after defeating Commander Saturn and travel northwest to Lake Verity. You will be approached by Professor Rowan who asks you to assist Dawn.
Travel through the path to locate Dawn. You will be attacked by Team Galactic Grunts along the way, however the battles can be avoided by using Surf to cross the water areas. At the end of the path you will find Dawn and another Team Galactic boss, Commander Mars.
Defeat Commander Mars using your highest level Pokemon. Commander Mars also uses a Golbat and Bronzor, however she also has Purugly, which is only weak against Fighting-type attacks.
Fly to Celestic Town then travel west to Route 211 and up to Mt. Coronet. The path to Snowpoint City is no longer blocked by the Team Galactic Grunts.