You just need Pokemon that are effective, or high level etc.
12334angel
After the elite 4 (in Pearl Diamond or Platinum) you need to go into the big room and find Cynthia and defeat her and go to professeur and get the national if you have seen all the Pokemon in the sinnoh dex
Cynthia IS the Sinnoh region champion, so to face her you will need to collect all the badges and defeat the Elite Four.
No, you need to defeat the gym leaders to defeat elite 4. After you have defeated them once, you cannot battle them again, unless you start the game over.
Yes, you need all eight badges to battle the elite four in Pokemon pearl.
You need to defeat the elite 4 and Cythia and then go to snowpoint city and you can go on the boat just down from there
To unlock the pal park in diamond/pearl and enable migration from other GBA games, you need to defeat the elite 4 and then get the national poke'dex by seeing all the Pokemon in the diamond/pearl Pokemon region.
You don't need to catch one, you just need to defeat the Elite Four and Champion then go to Bebe's house (the one beside the Pokemon Center at Hearthome City) and talk to her.
your Pokemon need to be lv 40 to beat him
You get Shaymin on the path that goes North-East out of victory road. This route will appear when you defeat the elite four. However you need an event to get Shaymin.
you need to defeat the elite four on both games
psychic, dark, flying, ghost
Defeat the Elite Four! Good Luck!!