How do you beat the elite 4 on diamond?

First you have to get to the elite for, past Victory RD. You have to use your strongest Pokemon. Once your there, Your rival will come to battle you once you're at the door. Usually it's Staraptar (beat it with Electric or ICE), Heracrossbeat it with Flying [X4 Effectivness], Psychic or Fire), or Snorlax (beat it with Fighting).