Toughest Elite 4 IMO is the Hoenn League (Based at every pokemon at LV 50)
The easy part of the Elite 4 is when you Over Level all of their pokemons :/
But what if everyone is at the same Level even yours?
First up is Sydney.
Not much Synergy in his team but hitting hard on every pokemon you have.
Second is Phoebe.
Her main role in the Elite 4 is to Drain your pokemon's PP using Spite, Grudge and Using the Pressure Protect Combo
Third is Glacia.
Her role is Endurance and slowly crippling your pokemon's life using Hail and Defensive pokemons
Her real threat is Walrein however. Besides it, you can just smack it with Fighting,Electric or Fire Types
Fourth is Drake.
His role is the same as Sydney's but his pokemon Sweeps like Crazy if he's able to Set up his Dragon Dancing Altaria or using Slamence.
Using Ice-Moves kills his cripple team however
And last is Steaven / Wallace
Their Pokemon has the same role but with Different Strategies
First is Wallace.
He's using, the ever so popular, RainSwim Team
Utilizing the Rain to his Pokemon's Advantage. using Ludicolo's Swift Swim ability to sweep teams with untrained IV/EVs. Wiscash for blocking electric type's attacks. Gyarados' Gen 3 Dragon Dance Set appears in his team(EQuake, DD, Hydro Pump, Hyper Beam). Tentacruel's high special defense leaves special attackers to a stand off between Rain Boosted Hydro Pumps or Toxic+Double team the boosters to death. Wailord is Wallace's lead poke, be wary of it's Rain Dance, Water Spout, Rest and Ice Beam. Getting hit by Rain Boosted Water Spouts isn't funny. And finally Wallace's Main pokemon and one of the bulkiest of his pokemon. Giving her a Status prob would let her defense rise by 50%. His Milotic has Recover, Toxic, Ice Beam, and Surf. Attacks like that are already good in Gen 3's Metagame.
Another Champion is Steaven
Using bulk as his strategy.
His lead is Skarmory. My favorite poke, Skarmory has been in the Metagames since Gen 2 and everybody know how bulky this pokemon is. Boasting a Reliable Defense stat spread and has a wide movepool to boot. It's attacks are Spikes, Toxic, Steel Wing and Sand Attack. The 2 fossils in 3rd gen, Cradily and Armaldo. Cradily has a very Defensive Movepool while armaldo has an offensive movepool. Despite having 2 of the types that has the most weakness,grass and rock, Cradily has only 2 weaknesses that consists of only Fighting and Ice. Aggron, even though he boasts Supremely high Defense, it's actually the weakest in his team. Having Fighting and Ground as its double weakness, which is very common for a pokemon team to have. Water kills this pokemon very quickly cuz of its very low special defense. Cradily is another defensive pokemon that will take awhile to defeat after it has set up its cosmic power and is very tedious to get rid off unless you roar/whirlwind it away reseting its stats back to zero. And Finally, Steaven's Ace, Metagross. Metagross is one tough pokemon to defeat with Meteor Mash, Psychic, Earthquake and Hyper Beam (Which is already powerful in that generation). having its stats nicely spreaded but has higher defense compared to its Special defense and its speed is its only drawback.