um, you battle them. which is a bit of a problem really because it takes ages to do and when your Pokemon get fainted then u start at the beginning. ive read about loadz of people having really high lvl Pokemon but i just caught Pokemon from victory road and i won with a lvl 61 dialga, 41 abomasnow, 45 pelliper, 45 tentacruel, 46 golbat, and 67 staraptor.
--IMPROVED ANSWER--
The first person, Aaron, is bug type. That means you should go with fire or flying.
He has a level 53 Dustox, 54 Heracross, 54 Vespiquen, 53 Beutifly, and 57 Drapion.
The second person, Bertha, is ground type. That means you should go with grass or water.
She has a level 55 Quagsire, 59 Hippodon, 57 Sudowoodo, 55 Whiscash, and 56 Golem.
The third person, Flint, is fire type. That means you should go with water or ground.
He has a level 58 Rapidash, 61 Infernape, 57 Steelix, 57 Lopunny, and 58 Drifblim.
The fourth guy, Lucian, is phsychic type. That means you should go with bug, ghost, or dark.
He has a level 59 Mr.Mime, 59 Girafarig, 60 Medicham, 60 Alakazam, and 63 Bronzong.
The champion, Cynthia, is harder to beat, she has no definite type, but your Pokemon from the other battles should manage.
She has a level 61 Spiritomb, 66 Garchomp, 60 Gastrodon, 63 Milotic, 60 Roserade, and 63 Lucario.
Your Pokemon should all be at least level 50. A good range is 60-80.
Also, make sure to have plenty of potions and to use them to full heal after every battle.
You can get them by winning contests or you can beat the elite four
You can find out how many times you have beaten the Elite Four in Pokemon Diamond by checking your PC in the game.
it is the elite four
nation pokedex
The champion in the Elite Four.
defeat the elite four
-mongmong908
kill the elite 4 noob
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.
the same
Beat Cynthia after Elite Four
erm......you battle them.