FireRed LeafGreen transfers to Ruby/Sapphire. Transfer Pokemon from Ruby/Sapphire (and the transferred FireRed LeafGreen Pokemon) into Diamond via Pal Park. Also you need transfers from Pearl and/or Platinum. Maybe SoulSilver and SoulGold too?
Alternate method:
Action Replay device.
No, you do not have to beat the Elite 4 to get the National Dex in Pokémon Diamond.
Get the National Pokedex by seeing every Pokemon in the Sinnoh Pokedex
To get the National Pokedex, beat the Elite Four.
yes
No, there is not a Pokedex after the National Pokedex in Pokemon Diamond Version.
you need to beat the elite four and then get the national pokedex for Oak.
press switch pokedex on the national pokedex and you will be using the sinnoh pokedex
bebe gives you an eevee after you have the national pokedex and have beat the elite four
You have to beat the Pokemon League and get the National Pokedex and go underground and dig for one.
You have to obtain the national pokedex (beat elite 4 and go to the lab of prof.)
The credits roll after you beat the Elite Four. However, I would classify the game as being completed after you have seen all of the pokemon in the Sinnoh Pokedex and the National Pokedex.
If you have the National Pokedex, Prof. Oak will be in a house in southern Eterna City.