Use the first Rock Climb ledge to reach TM41 (Torment), then climb back up and travel further down to find another Rock Climb ledge. Use it and follow the path until you reach the western wall of the chamber. Before you go upstairs, follow the path straight ahead. Press the rock to find a hidden nugget. Use the stairs to reach 2F.
Using Strength and Rock Smash, carefully move the boulder and maneuver your bike carefully in fourth gear (faster speed) to jump over the bike ledge. Then switch to third gear (slower speed) and jump over the bike ledge to find a Max Elixir. Now travel to the southern part of the room while smashing rocks and moving boulders (make sure to move those boulders very carefully to be able to progress successfully).
Use the nearby stairs to return to 1F, then quickly head back to 2F. You can now move the nearby boulder in a new direction that will allow you to access a hidden area. By going through the smashing rocks/movable boulders/bike ledges in this area you can find some more items including TM71 (Stone Edge). Now return to those stairs you just used to get back to 1F.
Use the nearby Rock Climb ledge and carefully follow the path to reach the stairs for B1F.
Using Surf head to the nearby trainer and then from the pool on the right, follow the path in the southern area to briefly return to 1F to find a Rare Candy. Now head for the north area and use Waterfall to climb the waterfall. Then head for the next waterfall and descend down it find TM59 (Dragon Pulse) before climbing back up.
Once back up follow the path to the west to find some stairs to return to 1F.
From the stairs follow the path until you see the path leading down and head in that direction. Now head left to another Rock Climb ledge and from their you can briefly return to 2F to find TM79 (Dark Pulse). If you go up the stairs near the old man, the path leads to a blocked-off doorway that is inaccessible until after you've defeated the Elite Four.
Near the stairs to the left is a path that leads to the Razor Claw which evolves Sneasel into Weavile. The Rock Climb ledge that is on the right hand cliff near the one you used earlier will lead you out of Victory Road and into the Pokémon League.
Before you challenge the Pokémon League, use the Dowsing Machine app on your Pokétch near the base of the waterfall to find the Sky Plate. Now, use Surf and Waterfall to enter the building. Before going to the receptionist make sure to have a very good well-balanced team. Also make sure that all the members of your team are trained around Levels 54-58 to be able to stand a chance of succeeding. Also stock up on Hyper or Max Potions, Full Heals, Revives, and Full Restores. Just before you get to the receptionist in front of the door leading to the Elite Four, Barry will appear and challenge you to determine who among the two of you is more worthy.
The main new change is that Barry's Munchlax has evolved into a Snorlax, which should help count Snorlax as seen in the Sinnoh Pokédex. Snorlax can be easily brought down by a powerful Fighting-type attack such as Cross Chop or Close Combat. Other than that the same weaknesses as before can be used to defeat the other members of his team. Afterwards Barry will leave convinced he is not ready yet if he keeps losing like this, but vows to be the one to defeat you.
Talk to the Ace Trainer and once he confirms you have the eight Sinnoh Badges you can then face the Elite Four, but just remember that if you lose after passing through the doorway, you must start over.
For dealing with Bertha's team if you have a fastGrass or Water-type, then you should have an easy time. Whiscash has a 4x weakness to Grass-types, Golem, Rhyperior have a 4x weakness to Grass and Water-types, and Gliscor has a 4x weakness to Ice-types. Exploit these weaknesses with attacks such as Ice Beam or Surf. Hit Hippowdon, her only single type Pokémon, with powerful attacks before it can hit back. While Gliscor, Golem, and Rhyperior know moves to cover several of their weaknesses, their low Special Defense can still be exploited.
Flint will thank you for helping Volkner earlier and then face you in a battle. Be careful when his Pokémon use Sunny Day, as it can raise their Fire-type moves and weaken the power of Water-type moves. His first Pokémon Houndoom should be taken down as quick as possible to help give you an early lead before Sunny Day is used. Flareon should not be much of a problem, though be careful with Overheat since it is very powerful when first used. Also its Will-O-Wisp can weaken your Pokémon's physical moves by half and cause minor damage each turn. Rapidash's speed can be a hassle, and its Solar Beam can cover its weaknesses, especially in harsh sunlight. Infernape is weak to Water, Ground, Flying, and Psychic, but it can cover Water- and Flying-types with Thunder Punch. His strongest Pokémon Magmortar has a pretty strong moveset and Special Attack, but a faster Pokémon with a strong Water, Rock, or Ground-type move could easily take it out.
Cynthia will thank you for your help on Mt. Coronet and in the Distortion World earlier before facing you. Cynthia does not follow a specific type so this battle will be trouble if your Pokémon's levels are not high enough. Spiritomb has no direct weaknesses, though a Steel-type can be quite effective. Roserade shouldn't be that much of a hassle if you get it down quick with Fire or Flying types. It would be recommended to use an Electric-type against Milotic since it has Ice Beam to counter Grass-types. Physical attacks, like ThunderPunch or Wood Hammer, tend to work better because they make Mirror Coat fail.
Use Fire, Ground, or Fighting-type moves to bring down Lucario. Togekiss can be brought down by an Electric-type but be careful of its high defenses. Garchomp, the Champion's strongest Pokémon, is far more dangerous than anything you have battled. It may have a 4x weakness to Ice-types, but its high physical Attack and Speed often lets Garchomp one-shot your Pokémon before you can move. If you can get it to use Giga Impact, then exploit the free turn while you have the chance.
After the battle Cynthia will praise you on the conviction you have with your Pokémon to become Champion. She'll then take you to the Hall of Fame where Professor Rowan will join you and all the Pokémon on your team are recorded into the Hall of Fame.