The thing is, the Elite Four, are the last things in the game so after you beat them you win. It's best to have all your pokemon level 50+, and have at least one ice pokemon because of the last elite four has dragon pokemon, and ice can beat their Dragon Breath.
if you have pearl or diamond version, go undergroung and get lots of revive or buy them along with lots of hyper potions or max potions. save before and after every person and heal you pokemon outside of battle if possible
