In the Pokemon games, the Elite Four are kind of the final boss to the games. In every game (including Pokemon Diamond) you will find them at the end of Victory Road.
kill the elite 4 noob
You can find out how many times you have beaten the Elite Four in Pokemon Diamond by checking your PC in the game.
defeat the elite four
it is the elite four
The champion in the Elite Four.
nation pokedex
Yes,you can trade with perl,diamond,platinum and soul silver.You can trade before beating the elite four.
The elite 4 are in the north-east corner of the sinnoh region
No, you do not have to beat the Elite 4 to get the National Dex in Pokémon Diamond.
erm......you battle them.
Beat Cynthia after Elite Four
After you beat the elite four.