Hello and welcome to the Hoenn Elite Four guide. Hopefully this will help you if you are having a hard time with it. Good luck!
--------------------------------------------------
Recommended Pokémon
(can use Pokémon from previous regions)
Gyarados
Gengar
Clefable
Honorable Mentions
How to get mentioned moves
Recommended Healing Items
Elite Four
Sidney - Dark-types
Mightyena, Shiftry, Zoroark, Cacturne, Sharpedo, Absol
You can set up with Gyarados or Clefable with two Dragon Dances or two Calm Minds, respectively. Heal up if necessary and proceed to sweep.
Phoebe - Ghost-types
Dusclops, Banette, Sableye, Chandelure, Mismagius, Dusknoir
You can set up with Gyarados, after two Dragon Dances you can proceed to sweep Phoebe's team. Heal up if necessary. Use Earthquake against Chandelure, otherwise you risk getting burned.
Glacia - Ice-types
Abomasnow, Froslass, Beartic, Vanilluxe, Glalie, Walrein
Set up with Clefable, after two Calm Minds you can proceed to sweep the whole team. You risk getting frozen by Abomasnow, however. Healing up against it might be necessary. Another option is using Snorlax, its Thick Fat ability weakens Ice-type attacks.
Drake - Dragon-type
Altaria, Flygon, Kingdra, Haxorus, Dragalge, Salamence
You can set up with Gyarados or Clefable, with two Dragon Dances or Calm Minds, respectively. You can sweep from this point forward. Kingdra can be problematic for Gyarados, you might need a Fairy-type to deal with it. Dragalge can be problematic for Clefable if it is faster or Clefable gets crit. Gyarados is able to revenge kill it with Earthquake, however.
Champion
Steven
Skarmory, Claydol, Aggron, Cradily, Armaldo, Metagross
Gyarados is able to set up easily on Skarmory, three Dragon Dances are enough. Watch out for Aggron's Sturdy, Cradily's and Armaldo's Ancient Power. If Gyarados is able to kill all 5 previous Pokémon, Metagross should die to a +3 Earthquake. Snorlax is capable of beating Aggron, Cradily and Armaldo, if Gyarados died. Heal up the Gyarados when Snorlax is on the field and try to Intimidate the Metagross to reduce its damage output. Snorlax should always be able to tank at least two hits from it.
Hope you were able to beat Hoenn's E4 with the help of this Guide. If you have any questions feel free to reply to this thread. Special thanks to @ShinyCelebi who helped me with this guide.