Where is a good place to train all of my lv50 Pokemon before the you battle the elite 4?
Ok, you need to have a good fire type or flying type for the first battle. I had Infernape and Staraptor with me and used flamethrower and wing attack the entire time. If you want you could even use an electric type for the winged bugs like Vespiquen or Dustox. Note: Drapion is by far his strongest Pokemon so have a good ground type with you. Garchomp is the best choice for the Elite 4 because he's Dragon/Ground and you will need both. Teach him Dragon Claw and Earthquake and have lots of ethers.
For the Second battle have your lead be a strong grass type. I used Roserade for the first two and then switched to my water type for the Rock/Ground and pure Ground types. Bertha is pretty easy, just grass and water using surf and giga drain/magical leaf.
The third battle is where things start getting somewhat challenging, for Flint's Rapidash use your Garchomp. If its speed is better than the Rapidash just use Earthquake but if not use dig. Rapidash is most likely going to use a 2-turn move and since it's faster it puts you at the advantage. (when the attack hits you will be underground). For the rest of the battle you can alternate your water type or ground type.
The fourth battle is by far the hardest of the Elite 4. Lucian leads with Mr. Mime, a somewhat easy, pure Psychic type so use the move Crunch over and over until you beat it. If you don't have a Pokemon with Crunch just use fire or water or dragon. (dragon works on most types but check the ones it doesnt work on). His Bronzong is the only really tough Pokemon he has, but you can use Flamethrower to make short work of him. Infernape is a risky Pokemon to use against him because of his weakness to Psychic type attacks, but if his Sp. Attack is strong enough and has a good level you can use him, but make sure he can OHKO the Bronzong or you may be in trouble. His Medicham is super easy. My level 58 Staraptor took it out with one wing attack. Flying is the way to go with him. The rest, just use crunch or another dark type attack.
Cynthia, the killer. She is the strongest trainer you will ever face in the game by far. She opens with Spiritomp which would seem strong but really isn't. Just use any elemental attack on it. Surf and Flamethrower and Dragon Claw are good choices. Gastrodon MUST be taken out with a grass type move. Garchomp needs to be taken out with an ice type or dragon type. I used my own Garchomp. Roserade can be beaten easily with fire or flying. Milotic is pretty tough but a strong electric move will beat it in a couple shots. With Lucario, use Psychic moves, Fire, or even Water. Infernape is the best choice because most of lucarios moves are 1/2 effective against him. Just use Flamethrower over and over.
The Pokemon you use are up to you but make sure you have a strong Dragon or Ice, Fire and Flying, Electric, Grass, and Water.
ry Places near Sunyshore and Snowpoint if you want you can train in Victory Road though since you can't rematch trainers in a cave that's not my recommendation. Otherwise you can train against the Elite 4 by using up all your money on items that you'd need for the future and the Elite 4 and go through beating the Elite 4 (If you're doing this for training it's recommended not to use items and using up all your money.)