It's really hard but you have three choices.
1. Fight Pokemon which the Pokemon that you want to turn to Lvl 100 is weak to. (as in types)It will give you more exp points.
2. Fight Pokemon which are higher level than the Pokemon that you want to turn to level 100.
3. Use cheats/Action replay.
just train them
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.
train... train...TRAIN...
Just train your Pokemon till they are all level 100 then battle your friends.
Well,Level 70-100 Is A Good Level To Beat The Elite 4.
You have to train each of your Pokemon in your main party to level 100. Then try to beat them. Or.... use a cheat code!
level it up
Go through the game and train your Pokemon on fights.Otherwise use exp share and go to Pokemon League Battles.
Train your pokemon.
Level 100 legitly.
Well, the level doesn't really matter, as long as it's not 100. In the end, all the EVs will add up the same.
You can't find level 100 Pokemon you have to train a Pokemon to that level yourself.