Starly.
u have to beat the nationaldex
You get the diploma from catching all Pokemon from Nationaldex and Hoenndex.
You have to see all Pokemon Pokemon. But not the ones on nationaldex.
# Migrate It(Nationaldex needed)
You actually begin the game with the national dex.
find all 210 Pokemon on sinnoh pokedex then visit profeser rowan
At least 60 or more.
Defeat the elite four and become Pokemon champion with 60 Pokemon recorded in your pokedex.
Wait until there is a mass outbreak of Beldum after you get the nationaldex
you don't need to catch them just see all the Pokemon in the hoenn pokedex
you get it when you see every single Pokemon in Diamond or Pearl, then go and speak to proffeser rownan
no, not necessarily, you do however need the nationalDex which you get for seeing all 150 Pokemon in shinnoh