Before you venture forth, we strongly recommend using the Light Up Secret Technique, obtained by traveling through Diglett’s Cave and speaking to Oak’s assistant on Route 2. You can get through without Light Up, but having it will make a world of difference.
Floor 1F (north)
Once your eyes have adjusted, start by heading east. Along the bottom, there should be a costumed trainer guarding the path leading south. Go past him for now and grab the Repel by the dead end. Now go back and fight the trainer.
Poké Maniac Ashton
This trainer has a Slowpoke that matches his costume–talk about dedication. Since Slowpoke is Water and Psychic, it’s weak to Grass, Electric, Bug, Psychic and Ghost. But be wary of using Grass and Poison-types because of Confusion.
Moving on, head south from Ashton, then east after the nearby corner. After a while, when you reach the next corner, follow the tunnel north, then climb down the ladder at the end.
Floor B1F (south-east)
Downstairs, you’ll be a fairly wide area. Go south, then west. There should be another Slowpoke cosplayer lying in wait.
Poké Maniac Winston
That ain’t no Slowpoke, but it is one very rare Kangaskhan! Because it’s a Normal-type, pretty much any type should be able to handle it, with Fighting-types performing exceptionally well. Just be careful of the turn one flinch from Fake Out.
Afterwards, proceed west. About halfway along, there will be a path leading north. Ignore it for now and continue west. At the end, grab the Revive from the floor. Then go back and head north. Halfway up this path, there’s a Fighting-type trainer looking around in a circle, with a Dire Hit next to him.
Black Belt Dudley
Well, that’s a surprise; this Black Belt prefers his Rock-types. As such, Water and Grass-types are the way to go, with Ground and Fighting being valid options too. Definitely put away your Flying-types!
After thrashing Dudley, follow the tunnel north-east until it widens. Next, there should be two paths going around a mound of rocks in the centre. Towards the south-west of the mound is a Hiker, while a Black Belt awaits near the north-east. Both can be avoided. Meanwhile, in the north-east corner is 3 Great Balls.
Allen is basically a better version of Brock. His Pokémon are all the same types: Rock and Ground. Therefore you can use a strong Water or Grass-type to easily defeat them all. You could even let your partner Eevee heal all of its wounds with Bouncy Bubble.
Black Belt Eric
Eric has a Fighting-type Mankey that can be quickly dispatched with Flying, Psychic and Fairy-types. Poliwhirl isn’t a Fighting-type, yet, but a standard Water Pokémon. You defeated Misty, so we don’t need to tell you its weaknesses, right?
Finally, to the north-west of the rock mound is another set of ladders that you should ascend.
Floor 1F (west)
Upstairs, there will be a young trainer to the right. He’s guarding the path towards the right side. If you want, you can go around the left side to avoid him.
Just a single Fire-type and thus easy pickings for a Water, Rock or Ground-type. Hopefully you have one of those–and you will want to have one for the Fire-type Gym near the end of the game.
Regardless, go along the lefthand-side path. Near the bottom, there’s a Hiker who alternates between looking left and down. That means you can sneak past him if you time it right.
You’ve probably seen a lot of Onix now–hit it with Water or Grass-types if you’ve got them, or Fighting or Ground. Sandslash is an evolved Sandshrew, but retains the same typing. Therefore it can be bullied by Water, Grass or Ice-types. Flying-types can avoid its best move.
Beyond here, the tunnel will become more spacious. Near the top, there’s another Hiker, but this one you’ll need to fight to get past. Before challenging him though, head over to the bottom-right corner to scoop up an Escape Rope. Now go back and say "hi".
Machop is as simple as simple gets. Combat it with Flying, Psychic or Fairy-types if you own any. Rhyhorn is yet another Rock and Ground-type, so is therefore immensely weak to Water and Grass, and weak to Fighting and Ground.
After defeating Claus, go down the nearby ladder.
Floor B1F (north-west)
Downstairs, cautiously head west. The path will branch towards the north and south. Go north first, towards a dead end. Here, you can battle another dubious cosplayer to reach a Full Heal.
Poké Maniac Cooper
Cooper has an evolved Bulbasaur. It’s still Grass and Poison, so the list of weaknesses remain unchanged: Fire, Flying, Ice and Psychic. Very soon, you’ll be facing the Grass-type Gym, so you’ll need to be prepared to handle Pokémon like Ivysaur.
Next, slowly head south until you see a female trainer standing near the left. Go over and battle her so you can get past.
Ace Trainer Sofia
For an expert trainer, Sofia starts off with a pretty tame Pokémon, although it is quite cute. Still, we’re in a battle, so show no mercy with your best Water, Ground or Rock-types. Kadabra is a step up. This evolved Abra is weak to Bug, Ghost and Dark. If you have none, go with something that’s not Fighting or Poison.
Past Sofia, head south first. By the corner, there’s a male Ace Trainer near the left. He will alternate between looking right and down. Should you choose, you can avoid a battle with him by running past when he’s not looking. At the far end, you can grab some Stardust.
Ace Trainer Charlie
Similar to Sofia, Charlie starts with an unevolved Pokémon: Diglett, which is weak to Water, Grass and Ice. Next, he’ll send in Flareon, one of Eevee’s many evolutions. This one is Fire, so send out your own Water, Rock or Ground-type if that’s an option.
Go back up towards Sofia, then head west to find a Black Belt. Don’t be fooled: this guy will look in various directions, so you can sneak past him if you’re good.
Black Belt Steve
Steve is a one-trick pony, preferring to use Fighting-type Machop only. Feel free to return the favour by sending in your best Flying, Psychic or Fairy-type. Failing that, anything that’s not Normal, Rock or Steel should be OK.
Past Steve, go south to retrieve a Super Potion, then head north to reach a ladder going up.
Floor 1F (south)
Upstairs, you’ll be in the final area of Rock Tunnel. Just a little more to go! Nearby, there’s a friendly man who will restock your Poké Balls if you’ve ran out.
Next, slowly head south. Near the right, there’s a Picnicker looking around in a circle. Then, a few paces south, there’s a Camper near the left doing a similar thing. If your timing is very good, you can avoid both at the same time.
Nidorina is an evolved Nidoran (female). It’s a bit stronger now, but still a Poison-type. As such, it’s weak to Ground and Psychic-types. Steel-types are also good since they’re immune to Poison. If you haven’t already, try to train one of these soon because there is a Poison-type Gym.
If you don’t know or hadn’t already guessed, Leroy has an evolved Nidoran (male). For most intents and purposes, there’s little difference between the male and females. At the very least, they have the same weaknesses, so refer to our advice from above.
Either way, try to grab the Super Potion near the bottom-right corner. Once you’ve gone past the pair, the tunnel will widen towards the west. Over here, go and grab the Stardust towards the north. Then carry on west. Soon, there will be brave Picnicker, but you’ll need to fight her to keep going.
Two simple unevolved Pokémon to deal with. Do note that Meowth has Fake Out, which causes a flinch on Turn 1, and Feint, which has increased speed priority. Vulpix has Will-O-Wisp to cause Burn and Confuse Ray to, y’know, confuse.
Further west, you can find the long-awaited exit towards the south. If you are desperate to leave, head that way. Otherwise, continue west for just a little longer. You’ll come to a dead end, where a precious Pearl slumbers.
If you’re lazy to evolve your Pokémon, there are Golbat and Graveler to catch. In addition, you can add the Ground/Rock Rhyhorn to your collection. Plus the Ground-type Cubone if you’re lucky and the Normal-type Kangaskhan if you’re very lucky! Finally, the fiery Charmander is a rare spawn here.