After you have completed the Mauville Gym, you can go west to reach Verdanturf Town, which connects to Rusturf Tunnel.
Go into the red building and talk to the person there to get a Contest Pass. Then talk to the person in the corner to get TM45 Attract.
Make sure to teach Rock Smash to one of your pokémon, then go into the cave in the north part of Verdanturf Town. This is Rusturf Tunnel.
Go west from the entrance and go outside through the door there. When you are outside, go east and talk to the man there. The Blackglasses are hidden a few squares to the left of him. Talk to the man after you find the glasses.
Go up the stairs to find an HP Up.
Go back into the cave.
Create a Shortcut
Go northeast. You will pass Hiker Mike, who has two level 16 Geodudes and a level 16 Machop.
Go north from the hiker to find a Max Ether. Then go west and use Rock Smash on the rocks to create a shortcut through the tunnel. The man will give you HM04 Strength.
Keep going west and leave the tunnel. Talk to the Devon employee to get a Repeat Ball. This ball works best on pokémon that you have caught or owned before. You can buy more in Rustboro City.
Afterward, go all the way back to Mauville City. From there, go north to Route 111.