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)
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
Magikarp will evolve into Gyarados.
Magikarp evolve at level 20.
Magikarp can evolve into Gyarados, starting at level 20.
Magikarp evolve at level 20.
A Magikarp can evolve into a red Gyarados. The Magikarp needs to be a shiny Magikarp in order for it to evolve into a red Gyarados. This is due to the red Gyarados simply being a shiny Gyarados.
Get the Exp. Share and let your Magikarp hold it. When you battle, use a strong Pokemon and Magikarp will get some Exp. That's how I got my Magikarp to Evolve.
A Magikarp should evolve at about lvl 40 on Pokemon diamond
Magikarp evolve at level 20.
Magikarp evolve at level 20.
Magikarp evolves into Gyarados at level 20
Magikarp evolves at level 20.
Gyarados doesn't evolve as it is the max evolve form of magikarp. Magikarp evolves into Gyarados at level 20.