The Pastoria Gym is home to Crasher Wake, the Water-type gym leader.
Go north from the gym entrance, then go west, then south up the stairs. To the east, you can battle Tuber Jacky, who has a level 26 Buizel. You get 312 Pokédollars for winning.
Go west and step on the yellow switch, then go east, then north down the stairs. From there, go east, then keep going north until you can't go any farther. To the west, you can battle Fisher Walter, who has a level 26 Barboach. You get 1040 Pokédollars for winning.
Go east and step on the green switch, then go south down the stairs. Go east, then south, then west, then south, where you will battle Sailor Damian, who has two level 24 Wingulls. You get 1056 Pokédollars for winning.
Step on the blue switch, then go south, then east, where you will battle Tuber Caitlyn, who has a level 24 Azurill and a level 24 Marill. You get 384 Pokédollars for winning.
Go north, where you can battle Fisher Erick, who has a level 22 Goldeen, a level 23 Barboach, and a level 24 Gyarados. You get 1056 Pokédollars for winning.
Go north and step on the green switch. Then go east, then south until you can't go farther south. Step on the yellow switch. Then go north over the stairs until you are walking in the shallow water. Go west and follow the path. You will battle Sailor Samson, who has two level 24 Shellos, and a level 24 Wingull. You get 1056 Pokédollars for winning.
Go north and step on the blue switch, then go east, where you can battle Fisher Walter if you didn't battle him earlier, then go north to reach Gym Leader Crasher Wake. If you want to leave the Gym and prepare before you battle Crasher Wake, you can use the rafts to the left or right. Otherwise, talk to Crasher Wake to start the battle.
Gym Leader Crasher Wake
Crasher Wake has a level 27 Gyarados, a level 27 Quagsire, and a level 30 Floatzel. You get 3840 Pokédollars for winning. You also get the Fen Badge, which allows you to use the hidden move Defog on the Pokétch. You also get the stickers that Wake uses, and you get some TM55s. They teach the move Brine.
Leave the Gym by walking over the raft to the left or the right of Crasher Wake.
Get Defog TMs
Before you continue, go east from the Gym and north through the entrance to the Great Marsh. Talk to the northernmost person behind the counter and pay to start the Safari Game. Then talk to the Ace Trainer near the entrance to get some TM97s. This will add Defog to your Pokétch, which will be helpful later.
Talk to the Grunt
After you finish the Safari Game (see above), leave the Great Marsh and go back to Pastoria City. Go east from there and talk to the Team Galactic Grunt to the east of the Pokémon Center. The grunt will run off to the east, so go east and talk to the grunt again. Your friend will arrive and challenge you to a battle.
Battle Your Friend
Your friend has a level 26 Starly, a level 26 Buizel, a level 27 Ponyta, and, if you chose Turtwig, a level 31 Monferno, if you choose Chimchar, a level 31 Prinplup, or if you chose Piplup, a level 31 Grotle. You get 3348 Pokédollars for winning.
You can heal up at the Pokémon Center if you wish, then go east to Route 213.
Go east through the route. You will find the Team Galactic Grunt at the beach. Talk to him and he will go east. Go east and enter Hotel Grand Lake. You can talk to the person behind the desk to heal up your pokémon. Go north to go back outside. Go northeast to find the Team Galactic Grunt again. Talk to him and he will go north. Go north down the stairs and talk to him again.
Team Galactic Grunt
The Grunt has a level 25 Glameow. You get 1300 Pokédollars for winning.
After the battle, go north and you will find Cynthia. Talk to her to get Secret Medicine to use on the Psyducks in Route 210.
Help the Psyducks
Open the map and fly to Solaceon Town, heal up at the Pokémon Center if you wish, then go north to Route 210. Go north through the route to find the Psyducks to the left of the café. Talk to the Psyducks and choose Yes to use the Secret Medicine.
Get an Errand
Afterward, Cynthia asks you to deliver an Old Charm to her grandmother in Celestic Town. Choose Yes to agree to go. You will get the Old Charm. Go north from here to reach northern Route 210.