Instructions
Travel to Pastoria City. You will find it in the southeast corner of your map in between Routes 212 and 213.
Enter the Pastoria Gym and challenge the gym leader, Crasher Wake, to a fight.
Defeat Crasher Wake. He will field three level-30 Water-type Pokémon.
Take Route 213 out of the city and fight every trainer you encounter along the way.
Speak with the Galactic Grunt each time you encounter him. He will run away after every conversation.
Chase the Galactic Grunt to the Hotel area. Once here, you can engage him in battle. The Galactic Grunt fields four high-20s Pokémon but their type is the opposite of whichever starting Pokémon you chose at the beginning of the game. Defeat him and Cynthia will appear.
Speak with Cynthia to receive the Secret Potion.