You get Strength by using Rock Smash on the rocks in Rusturf Tunnel blocking the path of the Man and Woman. When you do so the Man will automatically give you Strength. Once you get the HM Strength all you have to do is click it and select "Use" then teach it to a Pokemon that can learn it. To use it all you do is click on a boulder (with no cracks) and there ya go. You used it.
You must learn strength. A HM used to move boulders.
The HM04 Strength can be found in Rusturf Tunnel in Pokemon Sapphire.
The following Pokemon that you can easily get to learn Flash in Sapphire: Beautifly or Dustox Lotad or Lombre Oddish Gloom
unova
If you are looking for a hm slave that can learn all three, I would recommend marill. Marill can also learn waterfall and surf.
No.
It can learn it via the TM but not by level.
You can only clone Pokemon in Pokemon Emerald. Not Sapphire and Ruby. To learn how to do it, search it up for a guide.
strength
When you learn to use the force.
No.
yes,camerupt will learn fissure it will learn it at Lv.55