The only way I can think of is put them in the daycare.
keep battling untill the pokemon reaches lv 100
Have a Pokemon that was traded to you then have the Pokemon hold a lucky egg and start battling your Pokemon will level up very quickly.
just keep practising with the Pokemon league until your Pokemon goes to level 100. Or If you haven't got to the league yet you can keep battling trainers and wild Pokemon.
Train your Pokemon and level them up by battling.
You level up by battling wild Pokemon, Trainers, and Gym Leaders.
Not all Pokemon require you to battle and level up to battle. For those that do, Rare Candies can usually be a simple method of levelling up quickly.
You should buy a DS Action Replay. With that you can level up a lvl.1 to a lvl.100 in half an hour or less, with continuous battling.
I say just keep battling weak pokemon or normal ones
your Pokemon sould be at least a level 25
keep battling Pokemon and people
You level up your Pokemon, by battling around the sinnoh region, just like any other Pokemon game
By battling other pokemon, use rare candy, or by putting the in the day care for a while.(note: if you leave a pokemon in the daycare for a while they may forget some moves you taught them)