Elite Four

Just train. A good spot to train is the Resteraunt in Valor Lakefront. I beat the Elite Four with my highest level 53 (I picked Turtwig at the start) so you don't have to train too much. Or you can keep doing the Elite Four over and over again for training. Another idea is to stock up on healing items suck as Full Restores and Hyper Potions earlier (after you get your sixth badge is a good time). Hope this helps!

A good thing to keep in mind is the weaknesses of the Pokemon that the Elite Four (including the Champion, Cynthia) use.

The first member, Aaron, generally uses Bug pokemon. Using fire or flying attacks on his Dustox, Beautifly, Heracross, and Vespiquen are effective. Use a ground attack on his Drapion.

The second member, Bertha, uses only ground pokemon. I reccomend you use a grass attack on all her pokemon.

The third member, Flint, as you've in Sunnyshore City, uses fire attacks (more so than Pokemon, as he only has two fire pokemon). Use water or ground attacks on his Rapidash, Infernape, and Steelix. Use a fighting attack against Lopunny, and finally, use an ice (or rock, electric, dark, ghost) attack on Drifblim.

The fourth member, Lucian, uses Psychic Pokemon - but don't use any ghost attack! Using a dark attack on Mr. Mime, Girafarig, and Alakazam prove useful. Use a flying attack on Alakazam, and use a strong fire attack on Bronzong.

Now for the Champion; Cynthia. She uses a mix of Pokemon, and require some strategy. She sends out Spiritomb, who's weak against nothing, at the beginning of the battle, however using a Dark attack proves to be effective. Use a fire attack on Roserade, grass attacks for both Milotic and Gastrodon, a flying (or fighting/ground) attack on Lucario, and an ice attack on Garchomp.

Overall, I would suggest a team consisting of these Pokemon to tackle the Elite Four's weaknesses:

If you've chosen Chimchar as your starter, use Infernape as your fire and fighting pokemon. If not, then use a Rapidash (for fire) and Medicham (for fighting).

Sneasel/Weavile, Roselia/Roserade, and Garchomp would fill in your team.

That's it! You can fill in the last slot (or two) with a Pokemon (or two) of your choice.

Now, I would reccomend for your team to be somewhere between levels 50 and 60; however, going beyond that with a Pokemon is viable.

Focus on one Pokemon at a time. Train them in areas in which the Pokemon there are weak to the Pokemon you're training.

For example, while training Roselia, train in caves, which consists MAINLY of ground and rock pokemon.

Keep going at it like that; then, use the Vs. Seeker against trainers to re-battle them.

Once you think your Pokemon are strong enough, train each of them in Victory Road against any Pokemon you run into; don't run away, no matter what.

Then, buy some Full Restores and Revives, and go against the Elite Four!

Ok 1st elite 4 if you have diamond use dialga with that ancient power you can kill them all or a strong fire type like rapidash

2nd one bertha use water/grass types cause of quacsire and wishcash

3rd use water or ground/rock types because of drifbloom and loppuny kill it with the 2 types I told u
Your

4th make sure you have alot of revives cause you might have to bring your rapidash back to life use a dark type like lucario he can learn dark pulse and rapidash on bronzong

Champion
Spiritomb kill it with dark moves or dragon moves it's defense is kinda low
Roserade fire it will probably die depending on you're Pokemon lv
Electric type for milotic trust you take out a grass type first thing it will do is ice beam
Gastrodon use a grass type it will probably die in 1 hit
Lucario use a fire type or ground type
Garchomp use a dragon type or ice type

If I were you I would get these pokemon

Lucario
Rapidash
Dialga/palkia
A garchomp
Ur starter
Ur choice hope I helped!

Ok 1st elite 4 if you have diamond use dialga with that ancient power you can kill them all or a strong fire type like rapidash

2nd one bertha use water/grass types cause of quacsire and wishcash

3rd use water or ground/rock types because of drifbloom and loppuny kill it with the 2 types I told u
Your

4th make sure you have alot of revives cause you might have to bring your rapidash back to life use a dark type like lucario he can learn dark pulse and rapidash on bronzong

Champion
Spiritomb kill it with dark moves or dragon moves it's defense is kinda low
Roserade fire it will probably die depending on you're Pokemon lv
Electric type for milotic trust you take out a grass type first thing it will do is ice beam
Gastrodon use a grass type it will probably die in 1 hit
Lucario use a fire type or ground type
Garchomp use a dragon type or ice type

If I were you I would get these pokemon

Lucario
Rapidash
Dialga/palkia
A garchomp
Ur starter
Ur choice hope I helped!

Just figure out their types, then get Pokemon that are strong against them!

Hope I helped!