Head from Driftveil City all the way through Route 6 into Chargestone Cave . When you are moving past the Season Research Lab you’ll come upon Cheren. He’ll stop you and take you inside, where he’ll present you with HM03 - Surf !
This gives you the ability to surf on water, opening up a substantial number of areas in previous areas that you couldn’t access before. All of these areas are listed in the Surf Required Areas section. Feel free to check it out if you so wish to.
Head back outside. There’s actually four items that we can now collect in Route 6 before we continue on, so let’s collect them first! First, head back east along the path. Head over the bridge and south through the Tall Grass. When you reach the next bridge, use Surf to move onto the water south of the bridge.
You should see an item sitting on the land to the south-west of the bridge. Move onto the land and grab the Heart Scale . Surf back onto the bridge. Move north back through the Tall Grass east of the bridge until you reach the next bridge. Now use Surf on the water north of the bridge.
Move north and you’ll see some Tall Grass to your right. Move into it. There’s a gap in the Tall Grass in the bottom-right corner of the area containing a hidden Big Mushroom . Collect it, then grab the item to your north, a PP Up . Use Surf and head over the water to the land on the western side.
Move west and you’ll be back at the Season Research Lab. Now and continue on towards Chargestone Cave . When you cross the bridge north of the Parasol Lady moving around in the puddle, a wild Pokemon will suddenly appear, scare your character, then disappear. An old couple will appear behind you
Once they’ve left, head east to the house of the lady that heals your Pokemon. Head into the water just east of the house. Surf east along the water. When you’re behind the Season Research Lab, you’ll notice that there’s a strip of land there with an item on it. Move onto it and grab the item to find that it’s TM56 - Fling ! Awesome!
You can now continue Surfing east to the entrance to Mistralton Cave and enter and explore it. If you do, you’ll need to make sure that you received TM70 - Flash from the man in Castelia City first as the cave is extremely dark and difficult to navigate without it.
If you do wish to explore Mistralton Cave , skip ahead to that section, then return to this point in the walkthrough when you’re done. When you’re ready, continue to the entrance to Chargestone Cave and enter it.
Chargestone Cave 1F
Head through Chargestone Cave to the point where Bianca and the Worker were talking just before the bridge. They will now have left and you’ll be free to continue along the bridge. Do so and you’ll be stopped half way with someone calling out to you. Once they finish, continue on to the other side.
Push the large crystal south into the huge crystal to unblock the path, then move south. Follow the path until you come upon a trainer. He challenges you to a Triple Battle! Get ready, because this one can be painful!
Triple Battle: Ace Trainer
Once he’s beaten, continue past the staircase leading down, around to the two large crystals. Push the bottom one west into the huge crystal, then move through the gap that you’ve created. Move to the very edge of the platform jutting out from the cliff and grab the Magnet . Move back to the staircase and go down it.
Chargestone Cave B1F
Follow the path along until you come upon two people. Talk to either of them to receive a Nugget and a Big Nugget too! HELL YES! Head west. You’ll reach stairs leading back to 1F. Go up them. If you jump down the ledge next to the steps, you’ll be back at the start of the cave.
Talk to the woman above the stairs, then head south to the Pokemon Ranger looking for a battle.
Trainer Battle: Pokemon Ranger
Head south and turn your Dowsing MCHN on. Use it to find the Carbos hidden beneath the floating crystal. There’s nowhere to go but back. Head back to the two guys that gave you the Nuggets. From the two guys, move north up the steps. Take the western-most path.
Turn on your Dowsing MCHN and use it to find the hidden Ultra Ball . Continue until you reach a Doctor. Battle him.
Trainer Battle: Doctor
You’ll be healed after the battle and be able to talk to the Doctor at any time to heal your Pokemon again. Head east and grab the Iron along the path. You’ll find a Pokemon Ranger to your south. Move over and challenge her.
Trainer Battle: Pokemon Ranger
Head back to where the Doctor is then continue north. You’ll eventually reach a Scientist walk around in circle spinning about. Move into her vision to challenge her to a battle.
Trainer Battle: Scientist
From there, move down the somewhat obscured steps leading west. Push the first large crystal west and the second north. You’ll find Bianca here. Move over and talk with her, then move up the staircase.
Chargestone Cave 1F
Move east, then south. At the intersection you can move east and you’ll find the doorway leading out into Mistralton City ! Head there now if you want, or feel free to explore the rest of the cave first (there’s not much left to do). Don’t forget that you can heal yourself at Mistralton!
If you want to first explore the rest of the cave, move next to the girl moving east and west looking north and south. She’ll challange you to a battle
Trainer Battle: Ace Trainer
Now take the south path of the four. Continue until you reach some steps taking you to a Hiker standing in front of an item. Talk to him to challenge him to a battle.
Trainer Battle: Hiker
Behind him is a Carbos . Grab it then head back to the intersection. That’s really it! If you want to, you can take the path west which will lead back to the rest of 1F, including the entrance. If you do, you’ll reach a large crystal blocking your way. Push it west into the huge crystal.
Move through and head south down the stairs and you’ll be back near the start of the dungeon. Once you’re done, head back to the intersection and take the path east to the exit. Move through the doorway into Mistralton City !
The following section covers Mistralton Cave , which you had the option to cover earlier on, before you entered Chargestone Cave.