Like rep13 said, the elite four is probably one of the best places to train your pokémon. The only problem with the eliete four is that you need to be in a caliber of at least level 55 to actually "train" against them. Before the Elite four though, victory road is NOT a good spot because wild pokémon just can't compete experience-wise. If you can't fight the elite four. I would search around for trainers that would like a rematch and just fight them.
If you've beaten the elite 4 and the Champion, you can go to the tower where Rayquaza is found (East of Pacifidlog) to encounter level 47 - 50+ monsters.
oh yeah. I forgot about rayquazas place. and i have beat the elite four. my weakest guy is 51 and my strongest is either 53 or 54. the only problem is, my groudon and blaziken get the most action in the matches. my slaking is the one that i want to train the most and i cant do that in the elite four. i cant do that in rayqazas tower either. there is just no good place to train for a normal type with only one attack that will effect ghost type pokemon (thunderbolt). i cant teach slaking psychic and i dont know about shadow ball. im really thinking about getting rid of slaking for a claydol or some ghost/dark/psychic type pokemon. problems that have to do with anything other than training my slaking and other teamates, please post it on my small but strong team topic.
Rare Candy is a no-no! It would damage the pokemon's EV points. Try using it after you get the effort ribbon. The effort ribbon lets you use Rare Candy without wasting any EV points.
Rare Candy and the Daycare raise pokemon without them gaining EVs/Eps. If Rare Candy were buyable, I would simply find the perfect IVs, EV train, and then feed the pokemon Rare Candy until it reached lv100.