First you need to Get the level 100 you want to train to gain TONS of experience. I mean lots. Like have it beat all of the elite four by itself without changing pokemon. Do this about three times. Or go to the unknown dungeon and battle until your pokemon runs out of PP for all of it's moves. Then take the pokemon look at it's stats. Then deposit the pokemon in a pokemon box on Bill's PC. Take it back out. Now look at it's stats again and compare them to the stats you wrote down. At least some of them should be higher. If you do this with a rare candied pokemon they may go up by ten points or more. Average trained pokemon may only go up by three or four points. Well trained pokemon may not go up at all. It's well worth a try. My tauros gained seven defense points after six or so times through the elite four. Try this trick out and power up your pokemon!