put strong Pokemon on your team and a magikarp.
put magikarp first, and go to a place with high level Pokemon. when you meet a wild Pokemon, switch your Pokemon with a high level Pokemon and make it faint. that way, your magikarp will gain 1/2 experience. the other way is to put on experience share(it is an item)
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Magikarp will evolve into Gyarados.
Magikarp evolve at level 20.
Magikarp evolve at level 20.
A Magikarp should evolve at about lvl 40 on Pokemon diamond
Magikarp evolves into Gyarados at level 20