Pokemon platinum version.

Before you face the Elite Four, I would recommend you have plenty of Revives and Hyper Potions. I would also recommend your Pokemon Levels be between 55-60.

Having a Water-Type Pokemon (With an Ice-Type Move), Grass-Type Pokemon, Fire-Type Pokemon, Flying-Type Pokemon, Dark-Type Pokemon, and a Ground-Type Pokemon would probably be a good Team to face the Elite Four. However, most Pokemon Teams can defeat the Elite Four if their Levels are good and they have many Health Items.

Hope this helps!

You should train your pokemon. That is the solution to most problems in this game. Just train your Pokemon.

The recommended level for fighting any Pokemon is 3 levels higher.

Also try to have a good pack of medicine. I would suggest 12 Revives, 3 Full Restores, 2 Max Potions, and 2 Revival Herbs.
Though these are very expensive, it will benefit you in how much damage you have.

Hacking or cheating is a solution, but not a suggestion, as it causes guilt and your game could go "haywire".

Next, use Pokemon with good moves and types. Aaron uses Bug-type Pokemon so you should use Fire, Flying, or Rock-types to defeat his Pokemon. For Bertha, use Water, Grass, or Ice-types. For Flint, use Water, Ground, or Rock-types. For Lucian, use Bug, Rock or Dark-types, and for Cynthia, use Fighting or Bug-types.

Trade with friends for good Pokemon. They may be more experienced than you think!

Finally, just use an all-out DESTROYER team. One that has great moves like Draco Meteor, Hydro Pump, Fire Blast or Giga Drain. Also you may need moves like Synthesis, Confuse Ray or Imprison.

Tips 2 beat Elite 4:
Make sure your Pokemon are levels 55-60.
For Aaron (Bug user), use fire, rock or flying type Pokemon.
For Bertha (Ground user), use grass, water or ice type Pokemon.
For Flint (Fire user), use water, rock or ground type Pokemon.
For Lucia (Psychic user), use dark, bug or ghost type Pokemon.
Have them learn powerful moves (with a power of more than 90)
Examples of these moves for each type are:
Normal: Double-Edge-120
Fire: Fire Blast-120
Water: Hydro Pump-120
Grass: Leaf Storm-120
Electric: Thunder-120
Ice: Blizzard-120
Fighting: Close Combat-120
Poison: Cross Poison-90
Ground: Earthquake-100
Flying: Brave Bird-120
Psychic: Psychic-90
Bug: Bug Buzz-90
Rock: Rock Slide-90
Ghost: Shadow Force-120
Dragon: Draco Meteor-150
Dark: There is no move which has a power of 90 or more. The strongest dark moves are Crunch, Night Shade and Sucker Punch, all with a power of 80
Steel: Flash Cannon-90
Make your Pokemon hold items which boost the power of their moves. A list of various items are listed here for each type.
Normal: Silk Scarf
Fire: Charcoal
Water: Mystic Water
Grass: Miracle Seed
Electric: Magnet
Ice: Nevermeltice (note: this item will also cut down the power of fire moves!)
Fighting: Fist Plate
Poison: Poison Barb
Ground: Earth Plate
Flying: Sharp Beak
Psychic: Mind Plate
Bug: Insect Plate
Rock: Stone Plate
Ghost: Spell Tag
Dragon: Dragons Tail
Dark: Black Glasses
Steel: Metal Coat

Hope this Helps :-)