After using Waterfall to reach the upper part of Ever Grande City, you can go into Victory Road on your way to the Pokemon League.
Prepare
You will need to have pokémon with you that know Surf and Strength. You might also want to bring a pokémon that knows Flash.
Victory Road 1F
Go north in the upper part of Ever Grande City to reach the entrance to Victory Road.
Inside, Surf north, then go north and use Strength to push the boulder into the hole to the north.
Go north and then west, and battle Ace Trainer Albert, who has a level 45 Sigilyph and a level 45 Rhyperior.
Go up the stairs near the Ace Trainer and go west across the bridge to get a Full Heal. Then go back the way you came.
Go north from the Ace Trainer and go past the stairs along the way. Use the Dowsing Machine to find a hidden Max Repel at the end of the path, on a rock.
Go back the way you came and go up the stairs to the north. Go west and battle Ace Trainer Hope, who has a level 47 Frosslass.
Then go west, where you find a ladder down to the lower level. Go down.
Victory Road B1F
If you have it, use Flash to light up this dark area.
Use the Dowsing Machine and go west to find a hidden Ultra Ball on a rock.
Go southeast, then go west and push the upper boulder west, then go down and follow the path to push the next boulder east.
Go south, then east, then north, then go all the way east. You will find a couple of boulders. Push the lower one east and then go northeast to get the PP Up in the corner.
Go southwest and then south to reach some stairs. Go up and battle Expert Bryn, who has a level 45 Hitmontop and a level 45 Throh.
Go west from Bryn and go up the ladder.
Victory Road 1F, part 2
Go north and battle Ace Trainer Edgar, who has a level 47 Mawile.
Use the Dowsing Machine to find a hidden Elixir on the small rock north of the Ace Trainer.
Go east across the bridge, then go northeast and go down the ladder. Follow the path in the basement to find a Max Elixir, then go back the way you came and go up the ladder.
Go south and you can battle Street Thug Regan, who has a level 43 Houndoom and level 43 Shiftry.
Go south from the Street Thug to find another ladder. Go down.
Victory Road B1F, part 2
Go south. If you go west, you can have a double battle with Ace Duo Jude & Rory, who have a level 47 Armaldo and level 47 Cradily.
Go south and use Surf to go south to the exit. From there, go east to find TM29 Psychic.
Go back the way you came and go back into Victory Road, then Surf west to reach the next area.
Go north to battle Expert Theodore, who has a level 47 Sawk.
Go north and then push the southern boulder to the east, then push the northern boulder north twice, then east. Then you can go north to reach TM35 Flamethrower.
Go south from this area, then go northwest. You eventually find Ace Trainer Vito, who has a level 44 Electrode, a level 44 Darmanitan, and a level 44 Swalot.
Go northwest from Vito to find a Full Restore.
Go south from Vito and you can battle Dragon Tamer Egon, who has a level 47 Kingdra.
Go northwest from the Dragon Tamer and go up the ladder.
Follow the straightforward path here and you will have to battle Brains & Brawn Aden and Finn, who have a level 47 Musharna and level 47 Primeape.
Go north from the Brains & Brawn and go up the ladder.
Victory Road 2F
Cross the bridge, then use the Dowsing Machine to find a hidden Iron near the west side of the bridge.
Go north from there, crossing another bridge. To the left, there is a pool of water with a waterfall. You can use Waterfall to go up the waterfall here and find TM81 X-Scissor.
Follow the path the rest of the way and you will find Wally, who challenges you to a battle.
Wally
Wally has a level 46 Magneton, a level 46 Delcatty, a level 46 Roselia, a level 46 Altaria, and a level 48 Gallade that he Mega Evolves into Mega Gallade.
After Wally
Afterward, Wally gives you a Dawn Stone.
Go north to reach the Pokémon League. Be sure to get the Mewtwonite Y in front of the Pokémon League building, then go inside and heal up your pokémon at the Pokémon Center inside.
When you go north through the doors inside of the Pokémon League building, you will have to battle the Elite Four and the Pokémon League Champion. You won't be allowed back out until you have either defeated the Champion or all of your pokémon have been knocked out. You won't be allowed to change the pokémon in your party after you go through those doors.
Be sure to plan a strong team to use against the Elite Four. Buy plenty of healing items, and explore Hoenn to level up your team and find items and TMs that can help you.
When you are ready, go north through the doors of the Pokémon League to begin your battles against the Elite Four and Champion.