Well, in Emerald there's that cloning glitch, so clone all you rare candies, then feed it to one Pokemon to get it to level 100. If you want to, you can migrate your Pokemon to Diamond or Pearl. Or use Gameshark/Action Replay, or trade it with a friend and make them do all the work, plus make sure your Pokemon is holding Lucky Egg while with your friend. That's all I know, since I'm no expert at these things. You can also just keep it at the Baby or Basic stage, then just make it keep level up in that stage. Once it's at the peek of level 99 (or 98, depending), evolve it (if you want to). Then use Heart Scales or TMs/HMs to make it learn the moves that you want it to learn.
Or you can do a combination of the above. Or be smart and train your team like normal trainers.