@ yaya
Edit:
Go get yourself a Magnemite named "Legendary."
This Magnemite is a pretty big contender. And he's going to prove it. Now, before 2nd gen there was no Steel type (cry) so he can easily focus on electric attacks to show his stuff. So Legendary much know 2 moves with "Thunder" in the name asap. (Thundershock, Thunderwave, Thunderbolt, Thunder...?)
Now, Legendary knows that becoming a legend of Kanto is going to take a lot of patience. "Biding" his time, if you will. Yeah, that's TM34 for his third moveslot. Last move is up to you, but make it a good one :O
With a name like "Legendary" I think it's preeeeety obvious that he's got to do some legend killing. Good way to make a name for himself and all. So to evolve, Legendary must solo any one of the three birds. The remaining two birds must be soloed before you can compete in the Elite 4.
There's more to being a legendary Pokemon than just sheer power, though. It takes good PR skills. So after showing each bird who's boss, you need to talk to some people who might be impressed. Before heading into the Elite 4 you have to head back and show off your badass Magneton to Lt. Surge, then boast about beating Moltres to Blaine. Finally, having beaten Articuno, you need to show that all Ice types are wimps by soloing... err... what was her name? The Ice Elite 4 chick. (Not me making a joke, btw, I legitimately forgot her name XD)
Legendary must always be the highest level in your party (or tied for highest level). He must be in the lead position when your Pokemon are recorded as champions (so lead against Rival). And finally, since all other Magnemite/Magneton are competition, make sure he solos the crap out of all of them, and every finishing blow of said solo must be done by a "Thunder" move.
It was either this or a Magikarp that can't evolve and solos Agatha, just to make sure you'd suffer.