easiest way to train your Pokemon is by batteling each trainers or you can practice with wild Pokemon in the grass,cave,or sea
use the exp share
i recommend the restaurant near lake valor
the easiest way to train strength would be the Soul Wars minigame.
Train them, like a normal person would. Not only will you train up your Pokemon, but you can work yourself towards completing your Pokedex by having encounters with Pokemon you don't have/haven't seen before.
get a lvl100 graodon summon him after u summon the Pokemon u need to lvl up
i'd suggest that you'd battle more trainers. If you beat they're Pokemon you'll get a little boost in the xp than wild Pokemon battle
I would use rare candies. Or, train in Victory Road.
sky pillar and victory road is the fastest and easiest way to train but if you have good strong Pokemon plow through the elite 4 a couple of times
The easiest way is to give the Pokemon an experience share and train against high-leveled Pokemon. You could also just put them in the daycare and ride around on your bike. If you have access to red, put the experience share on the Pokemon and beat some of his Pokemon or the Mt. Silver Pokemon.
treats
Train
The best levels to have are 60 or above Pokemon wise i doesn't matter the easiest way to beat the elite four is train Pokemon that defeat each members Pokemon.