After making your way south through Route 14, you reach Route 15, which is split into a northern and a southern half.
The northern half of Route 15 can be reached from Route 14 by chopping down the tree west of the Ace Trainer in the south part of Route 14.
Go west from there, and you can battle Coach Trainer Midge, who has a level 40 Machoke and a level 41 Magmar. You get 4100 Pokédollars for winning. Afterward, you get TM31 Fire Punch.
Go west and you can battle Picnicker Yazmin, who has a level 37 Tangela and a level 37 Seaking. You get 740 Pokédollars and 2 Great Balls for winning.
Go west from there to find TM47 Surf.
Then go south and jump down the ledge.
Starting from the west side of the route, go east and you will battle Bird Keeper Edwin, who has a level 39 Dodrio. You get 2340 Pokédollars and 3 Poké Balls for winning.
Check in the middle of the grass north of the Bird Keeper to find a hidden Golden Nanab Berry.
East of there, you will battle Beauty Lana, who has three level 37 Growlithes. You get 2220 Pokédollars and 3 Poké Balls for winning.
Go east to find a Hyper Potion.
East of there, you can battle Punk Guy Alex, who has a level 38 Arbok. You get 1064 Pokédollars and 3 Poké Balls for winning.
East of that, you can battle Punk Guy Ernest, who has a level 38 Primeape and a level 38 Muk. You get 1064 Pokédollars and 3 Poké Balls for winning.
East of the Punk Guys, there is a set of 3 Ultra Balls.
Now you can go west into the Gatehouse. Go up the stairs, and if you have caught at least 50 species of pokémon, talk to Professor Oak's assistant to get an Assistant Set for your Clothing Trunk.
From there, go west into Fuchsia City.