You can cheat using action replay or just encounter many magikarps
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catch a magikarp and evolve it
tentacool and magikarp ... As you can also catch goldeen on emerald
At Lvl 22 I think.EditActually, it's level 20.
In Pokémon Emerald, you can catch a wild Gyarados by fishing for one in Sootopolis City additionally you can catch wild Magikarp in all bodies of water with the Old Rod fishing item and then after catching Magikarp you can raise it until it reaches the level of Level 20 at which point Magikarp will evolve into Gyarados.
You can evolve a Magikarp or fish for one with a Super Rod inside of Sootopolis City.