Take them to the elite 4 and battle there.
Train it.
Give the Pokemon a soove bell and train it such as if its level 5 train it against Pokemon level 2,3, and 4
take your low level Pokemon to the Pokemon league and make shure they are first in your party then as soon as the get sent in you switch out to your level 100 giratina or something
The best way to defeat them is to train your Pokemon. No Pokemon will be above level 50 or something like that. The 4 main elite four, their Pokemon are only in their forties.
Almost everyone's Pokemon is that way if you don't wanna get BURNED, THEN TRAIN!!!!!!!!!
The best way to train a ground type Pokemon is to battle it near fire or electric Pokemon. However, if you have an experience share, give that to your ground type Pokemon, and have your highest leveled Pokemon fight the highest level it can easily battle, and do it a lot, and experience points should flood in. However, if your highest level Pokemon is not over 50, don't use the experience share technique.
there is only one good way to beat a pokemon league in any pokemon game. that is without cheating. for me the best way without buying anything or cheating is to train ur pokemon to level 65 and up. i got through with pokemon level 55 and up but it was harder.
train all of them they are all useful in their own little way
You can train your Pokemon anywhere. There are many places to find wild Pokemon the appropriate level for your Pokemon. Various Pokemon trainers and gyms will help you along the way.
A good place train is the route next to Undella town. You can put exp. share on a weaker Pokemon let a stronger Pokemon do all the work and you will level up quickly because the Pokemon there are usually about level 50-60. Another way is by using the daycare to level up fast by just using your bike or running to level up.
There are literally hundreds of ways to get a level 100 pokemon. One way is by trading with a friend or through the GTS. Another way is to just train hard.
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.