When a Pokemon has the move Flash, you go into Party, then select the Pokemon with Flash using the 'A' button and there should be an option that says "Flash". It is only usable in dark locations or within battle.
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there are many MANY Pokemon that can use the hm flash in Pokemon sapphire. the following Pokemon can: grovile( treeko),kirlia(ralts)shroomish, eletrike, Roselia, plusle, munun, magnemite, voltorb and many more
The following Pokemon that you can easily get to learn Flash in Sapphire: Beautifly or Dustox Lotad or Lombre Oddish Gloom
Find an Abra to give flash in the cave in dewford.
Nincada is a fantastic Pokemon to use these moves on. Not only can it learn Flash and Rock Smash, but also Cut, Dig, and Secret Power which are all moves that are required later on.
the best pokemon to use is mudkip.