At the front of the Gym, you will see Flint, who notes that Volkner seems to be excited to battle you and reminds you to help reignite Volkner's passion for battling before departing for the Pokémon League.
Before challenging, make sure that your team is healed. The highest-leveled Pokémon you'll be facing is Level 50. Volkner specializes in Electric-types, which are weak only to Ground-type moves. Ground-types are also immune to Electric attacks, so use that to your advantage! Grass- and Dragon-types resist Electric attacks, but it's not a good idea to use Water- or Flying-types here.
Being the final Gym in Sinnoh, this one has is the most complex puzzle. This one involves hitting switches on the gear-like platforms to rotate them and make the correct path through. In the first room, hit the green switch on the platform on the right once. Then head for the switch on the left and hit it once to reach the next room.
In the second room, go for the nearby blue switch to the left and hit it three times. You can then cross the drawbridge near the stairs and get to the blue switch in the upper left hand corner. Hit that switch then the nearby green switch to get the drawbridge around the right wall ready so now you can go to the third room.
For the third and final room, hit the nearby red switch once and then the one that is nearby to the left to be able to pass the drawbridge on the left wall. Then follow the path to the green switch and hit it twice then hit the blue switch once and follow the path to use the drawbridge on the right wall and return to the first part of this room. Touch the first red switch you used again and backtrack past the right drawbridge again and you'll find the red switch moved the revolving platforms to where you now can reach Volkner.
While all of Volkner's Pokémon are weak to Ground, some are equipped with moves to cover their weaknesses. Most Ground Pokémon will have no problem with Jolteon, but Raichu's Focus Blast and Signal Beam might be threatening. Unless you can take out Luxray quickly, do not use anything that suffers a dual weakness to Ice, such as Torterra, when facing Luxray. Volkner's star Pokémon Electivire has Fire Punch to cover Grass, Ice, Bug, and Steel. It may defeat one of your Pokémon with Giga Impact, but that prevents Electivire from moving the next turn.
Once you beat Volkner, he will be so glad that he was able to have fun that he gives you the Beacon Badge, which allows you to use Waterfall outside of battle. Now that you have all eight Badges, all of your Pokémon will obey you, regardless of level. He'll also give you TM57 (Charge Beam), a damaging electric type move that has a 70% chance of raising the users Sp. Attack by one stage. Now head to Jasmine again and your rival will appear and tell you that he wants to get stronger. Jasmine, who is touched by this conversation, will give you HM07 (Waterfall). With this new HM, you can now head north to the Pokémon League.
Now that you can use Waterfall (optional)
You can pick up several items that were not accessible before.
Route 223 should be a straightforward water route since all you need are Surf, Waterfall, and maybe an Electric- and/or Grass-type Pokémon. You can catch a Mantyke here, but remember that you need a Remoraid in your party for it to evolve into Mantine. At the northern end is the Pokémon League.
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. Examine 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 there 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.