give it a dragon scale, and trade it.
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Seadra can be evolved into Kingdra by using a water stone
seadra
Evolve seadra into kingdra by trading him when he's holding it.
You can evolve Seadra by trading it with the Dragon Scale
Have your Seadra hold a Dragon Scale, then trade it with a friend. Seadra will then evolve into a Kingdra.