Pokémon Ruby and Sapphire/The Elite Four

These guys are the last bosses. After defeating them, you face the champion. (In Red, Blue, Yellow, Fire Red, and Leaf Green, this is your rival, who I will refer to as Gary. Gold, Silver, and Crystal have Lance as the champ. In Ruby and Sapphire, you fight Steven. You battle Wallace in Emerald.) I will cover the Elite Four from Ruby and Sapphire here.

The first of the elite four is Sidney. He specializes in dark Pokémon. He has an absol, a Mightyena, a Shiftry, a Sharpedo, and a cacturne. The Shiftry knows Extrasensroy, so beware!

Fighting or Bug Type, will do well against all of the Dark Types.Swampert and Blaziken is a fine choice as well. Also, if playing Ruby, the legendary Groudon is nice. Its fire moves will work well against Cacturne and Shiftry. Ground moves (Earthquake, especially) can knock out Sharpedo.

She has a Sableye, Two Dusclops, and two Bannetes. Dusclops knows Ice Ball, so keep Rayquaza far away from it. Earthquake attacks work on this team. Ghost or Dark Types that you have, will Win.
 * After Sidney comes Phoebe, a user of Ghost Pokémon.

She has Two Glalies, Two Sealeo, and a Walrein. Make sure you have a strong Kyogre, because Walrein can use Sheer cold! The best you can do is bring some hefty Electric Types. Leave your Fire Types.
 * Next up is Glacia, with Ice Pokémon.

He has an Altaria, Shelgon, two Flygon, and a Salamence! If you have a good Ice-Type, it will beat this crew!
 * Drake, the Dragon Master, is the final of the four.

Beware the Skarmory, Aggron, Claydol, Cradily, Armaldo and Metagross! Get Fire Pokémon! Face Skarmory with Fire Pokémon, Aggron with Ground Pokémon and everyone else with Water Pokémon.
 * The champion,  Steven Stone, has mostly Steel Type Pokémon.