After the Plasma Frigate, Cheren tells you to go to Route 6. At the end of the route you find Chargestone Cave.
Explore Chargestone Cave
Just north of the entrance to the cave you see Bianca. Go up to her. She explains that you can push some of the floating stones toward the electric rocks.
After she leaves, go north. Push the floating blue crystal to the left. Then go west and south to get a Full Heal. From there, go north.
There is a floating blue crystal on the left. Push it to the right. Go north, then go east up the stairs. Scientist Ronald has a level 30 Magneton. Go south and push the upper blue crystal to the right, then go south to get the Max Repel.
Go north then west down the stairs, then north to fight Guitarist Anna. She has a level 30 Emolga. Use your Dowsing Machine in this area to find a hidden Yellow Shard. Go east to find a Hyper Potion. To the south there are two floating crystals. Push the top one to the left, then push the bottom one down.
Continue along the path to the south, going down some stairs and then going east. Soon you reach a fork in the road. If you want a Hyper Potion, turn on your Dowsing Machine and go north. Otherwise, go south. Cross the bridge. As you cross, you'll hear someone expressing fondness for Chargestone Cave because electricity can be expressed with formulas. The person wants to stop a friend in order to save that friend. The person leaves.
Continue crossing the bridge. Push the floating rock south. To the east you will encounter Ace Trainer Corky. It's a triple battle. He sends out a level 33 Sandslash, level 33 Unfezant, and level 33 Klink.
If you go past the stairs near the Ace Trainer, push the bottommost stone to the left and keep walking to the left to find a Magnet. Now go down the stairs.
Chargestone Cave B1F
At first, the path is straightforward. Just keep going until you see two people. Talk to either one of them to get a Nugget and a Big Nugget. Save these until you have beaten the game so you can sell them for a big profit.
You can go west for a shortcut out of the cave, but it's one-way. Before the one-way shortcut, you will find a woman who unlocks Funfest Mission "Find Mysterious Ores!" South of her, fight PKMN Ranger Louis's level 34 Ampharos to get a Chesto Berry. Turn on your Dowsing Machine and go south from him to find a Carbos. Then go back to the guys who gave you Nuggets.
Go north from the Nugget guys, then use your Dowsing Machine in the narrow northwest path to find an Ultra Ball.
Go north toward the big electric rock. Nearby, there is Doctor Kit who has a level 32 Solosis and a level 32 Gothita. After you defeat him, he heals up your pokémon and will give you a free heal-up whenever you talk to him.
Go east from the doctor to find an Iron. South of there, you find PKMN Ranger Briana. She has a level 33 Minccino and a level 33 Excadrill. She'll give you a Chesto Berry.
Go north from the doctor. You'll encounter Scientist Lumi, who has a level 32 Joltik and a level 32 Golbat.
Use the stairs near the scientist, then push the bottommost rock to the left. Go west and get the Revive. Go back and push the upper rock north.
The stairs to the left are a dead end, but you can get some items (Escape Rope, Thunderstone, Metal Coat, Electric Gem) and fight some battles (including a Rotation Battle) if you go that way.
Chargestone Cave B2F
First you'll encounter Guitarist Beverly, who has a level 33 Zebstrika. South of her, push the floating rock to the right. Down below and to the left, there is an Escape Rope.
To the east, Ace Trainer Shaye is walking around. He has a level 34 Archen and a level 34 Electabuzz. North of him, you can find a Thunderstone.
To the south, there is Ace Trainer Vicki, who challenges you to a rotation battle with a level 33 Stoutland, a level 33 Krokorok, and a level 33 Ferroseed.
Down the stairs, you'll find Hiker Jeremy, who has a level 32 Onix and a level 32 Boldore. Keep going until you find a Metal Coat. Use your Dowsing Machine and go farther to find a hidden Electric Gem.
Now just go back upstairs.
Chargestone Cave B1F Part 2
Use the stairs east of Bianca to go back to 1F. Up ahead, there is Ace Trainer Mary. She has a level 34 Tirtouga and a level 34 Magmar. Go west from her and push the floating rock out of the way to make a shortcut to the entrance of Chargestone Cave.
Go south from the Ace Trainer. Down some stairs, you will find Hiker Otto, who has a level 32 Aron and a level 32 Nosepass. East of the Hiker is a Carbos.
Go east from the Ace Trainer to reach the exit. Outside you find yourself in Mistralton City.