you need to battle every trainer you see
Not Every Single one
Every kind of Pokemon there is.
cheats
A Pokedex cheat
There is no code for that, unfortuneatly.
To catch every Pokemon, you need action replay for the few last diamond legendaries. You will need Pokemon fire red/leaf green and emerald. Migrate them to Pokemon diamond. In Pokemon diamond, fill up your pokedex by catching Pokemon normally or with poke radar, or use gts. It will take a long time to catch all Pokemon
you cant get every Pokemon be4 you get the national dex
No, you cannot see every Pokémon in the National Dex in Pokémon Diamond however you can see every Pokémon in the regional Sinnoh Dex in Pokémon Diamond.
www.pokemon.marriland.com.
in stores and on the ground.
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You have to have seen or caught every Pokemon in the game