You are given a TM when you will need it.
Flash is an initial move learned by Volbeat in Generation IV. For the Pokemon who can learn Flash through TM70 in Generation IV, see http://bulbapedia.bulbagarden.net/wiki/Flash_(move)#Generation_IV_2.
Use the HM Flash this is given by the sage at the top of sprout tower.
Most electric types can learn flash like voltorb and magnemite.
To get a Shedinja in Pokemon Heart Gold you must trade it from either Pokemon Ruby, Sapphire or Emerald. Jacko25
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