Here's my Party which usually helps me to defeat the elite four

My Party:
-Vapourean Lv.85 (Should be holding Quick Claw)
-Charizard Lv.79(should be holding Lax Incense)
-MewTwo Lv.79 (Should be holding twisted spoon, you can get twisted spoon from any phtchic pokemon, MewTwo was already holding a twisted spoon when I caught it)
-Dragonite Lv.74 (Should be holding Nevermeltice, you can get it in the icefall cave on Four Island)
-Jolteon Lv.81 (Can be holdign anything you want it to hold, mine's holding a king's rock)You probably should use max elixir to increase Jolteon's thunder PP.
-Extra Slot(You can bring a pokemon you want to train or evolve, I brought my Skarmory, give the pokemon a exp. share)

For Lorelei:
-Dewgong
(Use Jolteon first, then use Thuderbolt)
-Cloyster
(Use Jolteon too, Thunderbolt)
-Pilowswine
(Use Charizard, use fire attacks, I recommend Flamethrower which can be brought at Celadon City at the place where you play the slots, where you can redeem prizes)
-Jynx
(Use Charizard, Flamethrower would be the best choice)
-Lapras
(Use Jolteon, thunderbolt, or if your sp. att is not at its max, try thunder as Jolteon has low sp. att)

For Bruno:
-Steelix
(Use Vapourean, surf)
-Machamp
(Use MewTwo, Psychic)
-Hitmonlee and Hitmonchan
(Use MewTwo too, psychic)

For Agatha:
-Gengar
Use Surf
(Use Vapourean, this is where quick claw comes in handy, as Vapourean has low speed, it will halp vapourean to attack gengar first before it uses Hypnosis) It is good if you maxed out vapoyrean's speed with Carbos
-Misdrevaus
(Use Vapourean's surf too but Misdrevaus may still have health left and it will use thunderbolt on Vapourean but it's ok cos vapourean has very high hp so aft that, use surf again)
-Crobat and Arbok
(Use MewTwo, psychic, it's super effective)
NOTE: If any pokemon falls asleep due to gengar's hynosis, use the pokeflute unless your oppenent is asleep too then use awakening

For Lance:
-Gyrados
(Use Jolteon, thunderbolt of thunder if you're not confident that thundebolt might kill it)
-Dragonite[or maybe dragonair if you've just started]
(Use Dragonite, you should catch one with a naive nature as it's speed is very high or if you can't breed a dratini you caught until you get one with a naive nature and use ICE BEAM)Even though your dragonite has a lower level than Lance's, if you're fast enough, one ice beam can wipe put Lance's dragonite so it's good that you also max out your dragonite's sp. att)
-Aerodactyl
(Use Vapourean, one surf will make it faint but aerodactyl may be faster but not to worry, vapourean is strong enough)
-Kingdra
(Use Jolteon, thunderbolt or thunder,thunderbolt may not be enough to kill it, but if you dun want to risk missing with thunder then use thunderbolt)

For Gary
-Heracross
(Use Charizard, Blast Burn, or maybe flamethrower if you're confident that it will knock out Heracross)
-Blastoise
(Use Jolteon, I advise thunder as thundebolt may not be strong enough but if you miss, on and off, you should have saved the game aft you defeated Lance before you fight Gary)
-Tyranitar
(Use Vapourean, Surf or Hydorpump)
-Alakazam
(Use MewTwo, use swift[hope you didn't replace it] it may be weak but use it twice and you can knock out alakazam but don't worry you won't suffer any damage cos' alakazam's busy using calm mind)
-Exxegutor, sorry if I mispelled it
(Use Draco, Ice beam)
-Arcanine
(Use vapourean, surf or hydropump)

And that's all! You will now go into the hall of fame! and dun off the game till they tell you that it's save alr or all your efforts will be wasted

IMPORTANT!ls save before you go in the elite four and after you defeat each of the trainers so that if you make a mistake you can try again

And here are my pokemong and their attacks:
Jolteon:
-Thunderbolt (get it a the placewhere you collect prizes for playing slots)
-Thunder
-Double Kick
-Thunderwave
Vapourean:
-Surf
-Hydropump
-Aurora Beam (I'm letting it learn Ice beam soon)
-Bite
Dragonite:
-Outrage
-Ice beam(You can get it at the same place where you get thunderbolt)
-Thunderwave
-Wing Attack
MewTwo:
-Recover
-Safeguard
-Swift
-Psychic
Charizard:
-Flamethrower
-Slash
-Blast Burn
-Fire Spin

Hope I helped you

Not to burst your bubble, but everyone has their own party for the elite four. But good team anyway.

Agree. but its a good team, i think i'll check it myself..

Thanks

MEWTWO LV 100 PSYCHIC, FLAMETHROWER, ICE BEAM AND THUNDERBOLT! WINS W/ 0 HP DAMAGE EVERY TIME! PAIR W/ ANY POKEMON W/ RAIN DANCE TO COUNTERACT ANY WEATHER CHANGES AND YOU ALWAYS WIN!!!!!!!!!