to get darkia get action replay same for the god and the little grass pokemon to get palkia (pearl) or dialga (diamond) it will be at the end of mt.cornet
Arceus
No, but it might as well be.
Go to all the lakes after capturing diaga
you need to battle every trainer you see
Not Every Single one
Every kind of Pokemon there is.
cheats
There is no code for that, unfortuneatly.
A Pokedex cheat
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.
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