well it is very hard and you wont like this answer but the easiest way is to hav lots of legendarys and high level pokemon!
-mongmong908
No, you do not have to beat the Elite 4 to get the National Dex in Pokémon Diamond.
Beat Cynthia after Elite Four
After you beat the elite four.
Beat all the elite four and there champion
catch legendary Pokemon like giratina and heatran
beat the elite 4 dips hit
beaten down by close combat!
finish the nation pokedex
beat the elite four and shes the champion
you have to beat the elite 4
To get the National Pokedex, beat the Elite Four.