I would have to say the the Elite Four is the best way to level up your Pokemon.
Before battling any of the four, make sure you do the following:
- Activate the Exp. Share so it is ON. This allows Pokemon to gain experience, even if they don't participate in the battle.
- Give the Pokemon you are training a Lucky Egg (if you don't have one yet, go talk to Kukui in his house). This increases the amount of experience gained by 50%.
- Bring the Pokemon you are training's affection hearts in Pokemon Refresh to at least 2 (can be done easily by feeding it a single Rainbow Bean). This increases the amount of experience gained by 20%.
- Have that Pokemon you are training be the only receiving experience. This means all other Pokemon in the party are level 100, fainted, or the Pokemon you are training itself (I do not recommend battling the Elite Four with a level 1 Pokemon, or with only Pokemon in general).
Now I have done extensive research on what Pokemon are the best to take down the Elite Four with ease and efficiency.
I, myself, currently use:
Ability: Mold Breaker
EVs: 4 HP 252 Atk 252 Spd
Earthquake (Combined with Pinsir's Mold Breaker, can sweep all of Olivia's Pokemon in one turn, including Sturdy Probopass and Golem)
Return (Combined with Mega Pinsir's Aerilate, can sweep all of Hala's Pokemon in one turn, although Fake Out from Hariyama still takes away a turn)
Ability: Snow Warning
Item: Light Clay
EVs: 252 SpA 4 SpD 252 Spd
Freeze Dry/Ice Beam (Combined with Snow Warning, can sweep all of Kahili's Pokemon in one turn, including Sturdy Skarmory. Can also sweep three of Acerola's Dhelmise, Drifblim, and Palossand in one turn)
Moonblast/Dazzling Gleam (Can faint Acerola's Sableye and Froslass in one turn, although Fake Out from Sableye still takes away a turn)
With these two Pokemon, the first trainers are a breeze, taking a minimum of 22 turns and 1 in-between-battles switchout, with damage only received by Hariyama and Sableye's Fake Out, and Skarmory's Steel Wing. The challenger in always random, but can be swept depending how you battle them, including Kukui's Snorlax, in one turn.
Very Very Optional
Recently, I have discovered a Pokemon that can take down the Elite Four single all by itself: White Kyurem. With the Special Attack boost, as well as Turboblaze, it can take the Elite Four down in 22 turns, since Hariyama and Sableye still flinch, and 0 in-between-battle switchout. Since Turboblaze is basically Mold Breaker, White Kyurem can faint all three Sturdy Pokemon.
Nature: Timid so it can out-speed Crobat
Psychic for Hala
Earth Power for Olivia (only obtainable through Move Tutor in OR/AS)
Shadow Ball for Acerola (I would personally use Fusion Flare for more coverage, since Ice Beam takes out three out of five of Acerola's Pokemon, and can faint Froslass and Skarmory with ease)
Ice Beam for Kahili
So, I hope this helps in anyway. To summarize, Exp. Share on, Lucky Egg held, Affection up. Battle the Elite Four fast so you can fight it again faster for faster experience. White Kyurem is Delete Four.