If you have beaten Mauville Gym and have gotten HM ROCK SMASH, go to RUSTURF TUNNEL and clear out the rocks. The digger will grant you HM STRENGTH.
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you get hm strength and hm defog after defeating two gym leader.Fantina(defog) Byron(strength).
You can find a Fire Stone in the Fiery Path after you get the HM for Strength.
The way you get waterfall is by trading the waterfall HM from Fire Red Version or the other Version that have waterfall HM ( you can trade with a link cable or wireless adaptor)
It is Strength. You will get it A. From a gym leader or B. Look around Mauville.