After using Rock Smash to get through Route 111, you have to take a detour because of the desert sandstorm and go to Route 112.
Meet Your Neighbor
Your neighbor will be near the entrance of Route 112. Your neighbor will heal your pokémon and give you HM04 Strength. You won't be able to use it outside of battle yet. Then your neighbor leaves to go to Fallarbor to look for Professor Cozmo.
Explore the Route
In the grass here, you can encounter Machop and Numel.
To the west, there is Camper Larry, who has Taillow and Zubat.
North of him, there is Picnicker Carol, who has Gulpin.
Go up the stairs and you encounter Hiker Trent, who has three Geodudes.
If you go northeast, you can battle Hiker Brice, who has Numel and Machop.
Northeast of Brice, Team Magma/Aqua are blocking the entrance to the cable car to Mt. Chimney. So you will have to go to the west and enter Fiery Cave.
There is a big boulder that is blocking most of the inner part of Fiery Cave, so just follow the path until you are out of the cave. While you are here, you can catch Grimer, Koffing, Machop, Numel, Slugma, and Torkoal.
North Route 112
At the other end of the cave, you find the north part of route 112.
To the east, you encounter Street Thug Jaylin, who has Mightyena.
North of the thug, there are a couple of Razz Berry trees and a Persim Berry tree.
Go east to reach the north part of Route 111.