There is no "best" Pokemon to defeat the Elite Four. You can use any Pokemon you want, as long as they are strong enough to face them.
If you want to strategize, Lorelei uses Ice Pokemon, so Fire, Electric, and Grass types would be useful against her. Bruno uses Rock and Fighting types, so Water, Psychic, and Fighting types would be useful against him. Agatha uses Ghost types, so Psychic, Ghost, Bug, and Poison, would be useful against her. Lance uses Dragon types, so Ice would be very useful against him. As for your rival, you'll need a combination, there isn't just one type to defeat him.
Just use a combination of the above types.
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After you defeat the elite four on Pokemon sapphire you complete the pokedex and make all your Pokemon level 100.
You go and have a tug
In any Pokemon game, you must defeat the elite four and the champion.
No you must beat the Elite Four first
i think u have to defeat the other gym leaders then find groudon then its the elite four but remember you have to find all the Pokemon after elite four