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As with all Elite 4 battles, I recommend buying 30 Revives and 30 Full Restores. You'll have a better chance of winning with a lot of those items, and even more chance with Pokemon above level 50.

Aaron is the master of the Bug-type. Without Fire type moves, you'll have to rely on other powerful moves like Drill Peck or Rock Slide to land super effective hits. His Dustox will attempt to pioson you, so some Antidotes will be handy. Drapion is only weak to Ground-type moves.

Bertha uses Ground-type Pokemon, and is very easy to defeat; she is like the Bruno/Koga/Sidney of DP. Moves like Petal Dance, Giga Drain, or Surf will be very effective here.

Flint loves the Fire-type, even though he has only two of them. Rapidash and Infernape can easily be dispatched with powerful water moves like Surf, or an Earthquake would be best. Steelix is also very easy to take out with a Surf from a high leveled Pokemon. Lopunny and Driftlim should be hit with powerful moves like Surf and Earthquake too.

If you can get a hold of a Spiritomb, or any Dark-type Pokemon and give it Dark Pulse (or Crunch if possible), you should fare well against Lucian. Otherwise, use high leveled Pokemon with the move Crunch (Luxray, Carnivine, Floatzel, etc), or blow everything over with Surf. Mr. Mime will eb the most trouble due to the Reflect and Light Screen moves. He'll make the battle go longer since his ally's defenses will be raise.

Unlike the rest of the Elite Four, Cynthia is pretty damn easy. She has an array of types, and this allows you to switch Pokemon and not rely on just one pokemon to wipe out a whole team.

Spiritomb has no weaknesses due to its dual-type, but Earthquake and Surf are effective against it.
Gastrodon, because of its dual-type is very weak to Leaf moves like Petal Dance or SolarBeam. This is a perfect opprotunity to use Giga Drain with a Pokemon low on HP.
Lucario is weak to Ground moves, and Earthquake if very good to use. It is at level 63, not 66; many sites reported it at such a level.
Roserade has two types, and Psychic, Flying, or Fire moves will do very well against it.
Milotic is able to restore HP with Aqua Ring, and can hit you heavily with Ice Beam. Electric or Leaf type moves are recommended.
Garchomp is very powerful indeed. Because of its dual-types, it is weak to Surf and even more weak to Ice Beam. There is a very low chance it will survive a single Ice Beam. It should be taken out as soon as possible, though, as it is a heavy hitter.