Rock Smash

• Go to the house in the east side of Mauville City, to the right of the Pokemon Center, and talk to the man inside. He will give you HM06, also known as the Rock Smash move. Open your item menu and select HM06 to teach it to a Pokemon of your choice in your current party. One of your Pokemon will need this later for you to get Strength. Defeat Wattson, the gym leader in Mauville city, to get the Dynamo badge and earn the ability to use Rock Smash outside of battle. Use a ground-type Pokemon, such as geodude, to easily defeat Wattson.
  
  

Rusturf Tunnel

• Go to Verdanturf Town and walk to the wall on the north side of the town. Enter the tunnel to the left of the Pokemon Center. You will need to have your Pokemon with the Rock Smash ability in your party to retrieve Strength from this area. Go to the Pokemon Center beside this tunnel if you do not have this Pokemon in your party. Stand next to the PC on the right side of the counter and press "A" to access it. Choose the "Deposit Pokemon" option to remove a Pokemon from your party to create space for the Pokemon that knows Rock Smash. Select "Withdraw Pokemon" and then choose the Pokemon with Rock Smash to add him to your party.
  
  

Hiker Mike

• Walk north from the tunnel entrance until Hiker Mike challenges you to a battle. He has two level 16 geodudes and a level 16 machop. Fight his two geodudes with water or grass-type Pokemon attacks. These are very effective against geodude and will inflict extra damage against him compared to other attack types. Do not attempt to use electric-type attacks because they have no effect. Use flying or psychic attacks against Mike's machop to easily defeat him. You can use other attack types against Hiker Mike's Pokemon, but they will not inflict the additional damage of the suggested attacks, and in some cases cause less damage.
  
  

Get Strength

• Continue north after you defeat Hiker Mike until you reach a man and woman separated by rocks on your left side. Approach the rock that is underneath the woman with the man on the other side of it. Face the rock and press the "A" button. Select "Yes" when the game asks "Would you like to use Rock Smash?" This will break the rock and allow the man to reunite with his partner. He will thank you and then reward you with HM04, Strength.