Fact: Most Pokémon take as much EXP to train from level 1 to 79 as from 79 for 100. So at level 79, you're not 79% done, but only half way.
Anyway, I have Emerald so I now just mass-clone rare candies. But if you don't feel like doing it that way, or don't have Emerald, then there's two main ways.
The first is continuous E4 battles. I trained my first Salamence that way. If you can 1HKO all their Pokémon, and turn animations off, it takes around 15 minutes tops per run-through. My record is 12 minutes, I think.
Another way is to do secret base battles. This is only really viable if you have a few friends with high-leveled Pokémon or multiple games and GBAs. If you visit a friend's secret base after mixing records with them, you can battle a CPU trainer who uses the Pokémon they had when they mixed records. However, you can only do this once a day, and if you lose, you can't battle them again that day even if you reset the game. It also doesn't work unless you've beaten the E4 already, which I assume you have.
On the positive side, if you can get 5-10 secret bases using Pokémon of levels 80 and up, you can level your own Pokémon quite fast. It helps if you get them to use Pokémon with high base EXP rates, such as starters, legendaries, and Chansey/Blissey.
Finally, there's a trick to battling is secret bases more than once a day - mix records with someone. Every time you mix records, it resets the bases, meaning you can battle all the secret base trainers, mix records, battle them all again, and repeat. Of course, this is only viable if you have multiple RSE games, multiple GBAs, and a link cable.