In Pokemon Platinum, go to the Resort Area (South of Route 229) and use the Super Rod in the pond by your villa. There's a small chance you can get a rare, wild level 100 Magikarp. (The level ranges from 1 to 100.)
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It's a level 100 Magikarp.
Your best bet would be to use the "all Pokemon shiny" code for your game then go fishing in the Resort Area for the level 100 Magikarp that can be caught there. In Platinum, that is.
I think level 100 but why don't you just go catch a gyradoas?
Magikarp evolve at level 20.
Magikarp evolves into Gyarados at level 20.