Catch Alot Of Diffrent Pokemon,Pokemon Global Trade Center,Migrate, And Just Try Really Hard.
catch them
no
To finish the sinnoh dex (Pokemon platinum's,diamond, and pearl dex) you just need to see every Pokemon not catch every Pokemon.
You can get Pokemon from the older versions on Pokemon Platinum, but I'm not sure if it includes every Pokemon from every game. You'll need to finish your Pokedex, then get the National Dex.
every pokemon from that game and generations before
battle trainers or catch the Pokemon
no
by using action replay
go to serebii.net
You see every single Pokemon, Google this "How to see every Pokemon in platinum DS"
No, you can't. Codejunkies has to create different cheats every game. If you want to get Pokemon on Platinum, then use the Platinum version of Pokesav.
You have to battle them.
yu have to get every Pokemon on the sinoh dex