How do you get Metagross in Pokemon Platinum?

In Pokémon Diamond/Pearl/Platinum there is a swarm in Route 228 that you can find wild Beldum. Be prepared, Beldum, along with its evolved forms, are the only non-legendary Pokémon to have a catch rate of 3, which is the lowest possible, making Beldum extremely difficult to capture due to its self-damaging move, Take Down.