You need Hm Rock Climb, you need to obtain all 16 gym badges.
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you get it after catching or defeating red gyarados
The dragon scale evoles seadra to kingra.When you trade seadra holding a dragon scale it will evolve to a kindra!
you go to the lake of rage and Battle the SHINY gyrados and then you will get it
In SoulSilver, Mr. Pokémon will give you the Exp. Share in exchange for the Red Scale that you had received after defeating or catching the Red Gyarados.
Have your Seadra hold a Dragon Scale, then trade it with a friend. Seadra will then evolve into a Kingdra.