if you have already beaten the elite four than you can keep going there and training you Pokemon there.
hope this helpz
give the Pokemon you want to train an exp share then put your strong Pokemon first on your team and train on high leveled Pokemon the Pokemon with the exp share should level up by itself!
There really isn't a quick way to get to level 100 fast without cheating. The best way would be to re-battle gym leaders and the elite four, along with wild pokemon. Sorry, you'll just have to be patient.
There isn't really a specific time to how fast your Pokemon level up. The younger the Pokemon(smaller the level) the faster they level up. also battles the Pokemon is in give it more experience. so if you wanted to level up a oddish and you gave it exp. share it would get less experience than if it was the first in the party/ battled the enemy Pokemon itself. If you trade what you want to train to a friend and give the Pokemon a lucky egg it well level up really quickly, only a few hours to wait and it will be level 100 in no time.
Mii Channel
cheat
In my opinion, I always goto the elite four (aftergame) and replay them. You can also goto the east side of Unova and fight wild pokemon.
They have clocks on the game. It depends on what time you set it as. It usually goes really fast- about 30 minutes for every minute in real time.
Leave it in your nursery. then you will not have to carry it in your inventory.
There is not really a easiest pokemon to evolve...but bug type pokemon evolve really fast
The Pokemon magikarp evolves into gyriados at level 20 just put an exp share on the magikarp to hold and the levels will shoot up really fast
try rare candies
Pokemon day care or put exp.share on a weak Pokemon