Instructions
Proceed through the game until you've defeated the Elite 4 and gained the National Dex. Once you have the National Dex, the game will start unleashing swarms every day starting at midnight.
Check an in-game TV to get the Pokemon Swarm report. Since there's a new swarm every day, check the TV daily until you receive a report about Magnemites swarming in Fuego Ironworks.
Travel to Fuego Ironworks. It's west on Route 205. Once you're there, wander around until you encounter a Magnemite. Your odds of encountering one are 40% for each fight, meaning that they're easy to find.
Weaken the Magnemite with basic attacks. Do not kill it. Once its health bar is yellow or red, throw a Pokeball at it to capture it. If you miss, throw more Pokeballs.
Place the Magnemite in your party and then start fighting battles with it as the team leader. It will gain experience and levels, and at level 30, it will evolve into the Magneton.
Go to Mount Coronet, which is between Routes 211 and 216. Once again, fight random battles to gain experience with the Magneton. If it gains a single level while on Mount Coronet, it will transform into a Magnezone.