Instructions
Talk to the Team Galactic Grunt who is standing outside of the Great Marsh Safari Zone area. He will run away. Follow him by heading east and then talk to him again. He will run away again and your rival will show up to battle you.
Defeat your rival using your strongest Pokemon. Your rival is below level 30 during this encounter, so pick your highest level Pokemon. Your rival also chooses Pokemon that are strong against your starting Pokemon, so plan your team accordingly. For example, if your starter Pokemon was a fire type, he will use water type Pokemon, so counter with grass type Pokemon.
Walk east from Pastoria City to find the Galactic Grunt again. He will be standing next to a fisherman on the beach. Talk to him again and he will flee. Walk east to find him again next to the restaurant. Talk to him and he will run away. Follow him to the north and talk to him one last time to battle him.
Defeat the Team Galactic Grunt using your best Pokemon and Cynthia will appear after the battle to give you the Secret Potion.
Fly to Veilstone City and walk west to Route 210. Talk to the Psyduck to give them the Secret Potion and they will vacate the area, giving you free passage to Celestic Town.