The only actual HM you need is Strength, although it would be useful to have Rock Smash to pick up a TM and a couple of other things. If you've beaten all 16 gym leaders and gotten Rock Climb from Professor Oak, you can also use that to grab an item or two.
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Go through the mystery tunnel then up the rider road
It is located somewhere in victory road
you have to beat all 8 gym leaders in joto before you can go to victory road
You can find an Onix in Dark Cave or in Victory Road in the main game.
rock smash water fall strengh surf and flash