In two words: experience points. Heaps and heaps of experience points.
As you might already know, Pokémon Omega Ruby and Alpha Sapphire include something called "secret bases". Think of them like treehouses. Hidden throughout the game are nooks and crannies that players can move into and decorate, Animal Crossing style. While the game gives you a variety of furniture and plushies to adorn your secret base, the best thing about secret bases is the fact you can can share them with other players. Anyone who downloads your base can then battle a digital version of yourself — which means you can set up something much like a gym for your friends to play in. Neat, right?
Some players are taking advantage of this feature to help other people level up quickly. As veteran players know, one particular Pokémon called Blissey gives players a crapload of experience points whenever it's defeated. Normally, you can't find too many Blisseys in-game, ...unless you set up a secret base with a triple-battle featuring only Blisseys. Naturally, players have taken advantage of secret bases in this way, and now there are QR codes that anyone can download for Omega Ruby and Alpha Sapphire with just Blissey battles:
It's fantastic. To give you an idea of just how much XP these battles can give you, after defeating ONE Blissey in these bases, my level 70ish Rayquaza gained a little over 19k experience. That means that, in a single battle with three Blisseys, my Pokémon can gain a little more than 57,000 experience points. And with the ability to import a whopping six bases through these QR codes, that means that my Rayquaza has a one-stop shop for gaining 342,000-ish experience points. That's MASSIVE — a guaranteed way to gain a bunch of levels without much hassle.
Thing is, the level 100 Blisseys in these secret bases don't have any attacks. Heck, they're even holding something called "Toxic Orb," which is something that badly poisons whatever Pokémon is holding it. Even if your Pokémon is a weakling, you shouldn't have too much trouble defeating the Blisseys in these battles — they're experience-points fodder.
OK, so, how do YOU take advantage of this great opportunity to power-level Pokémon? First, you'll need to make a secret base yourself. I'd recommend getting a secret base on the Secret Meadow, because it's a patch of land with seven secret bases, all in the same place. You can find the secret meadow here on the ORAS map, in case you're curious:
Once you're there, it should look a little like this:
Make a secret base in one of the little holes there. Go up to your PC, where you can import secret base QR codes. Import them using your 3DS's camera. You don't have to print them out — I just put the camera up to the code at the top of this post, and that worked.
Now, you can't battle any of the secret bases until a day after you import them. It's a drag, I know. But, if you'd like to kill them after importing them, I highly suggest getting a Lucky Egg. Lucky Eggs are items that increase the amount of XP earned on any Pokémon holding the Lucky Egg. Lucky Eggs are held by Pelippers, which you can find on these routes. I suggest capturing a few Pelippers so you can up your DexNav level enough that, when you click on Pelipper's image on the DexNav, the DexNav can show you what item the Pelipper is holding before you creep up to it. It took me about 20 encounters to find a Pelipper holding a Lucky Egg, but the experience gain is worth it.
Once you have the Lucky Egg in tow, go into the secret bases you imported. They're actually kind of nice: Before you challenge the person at the base, make sure to go up to this machine and pull the lever:
Then, go into battle with whoever awaits you at the base. Defeat the Blisseys and reap the rewards. Do note that any Pokémon not holding a Lucky Egg will gain more around the 13k XP mark for every Blissey defeated (assuming they're actually in the battle, and not just gaining XP from the Experience Share). That's still a sizeable experience boost, though! And finally, you can only challenge the people at these bases once per day. Don't think about changing the clock on your 3DS either; the game will know and punish you for it.
EDIT: As one reader pointed out to me, you can increase your gains here even further by activating the O-Power that increases the amount of XP you gain from battles.
With all of that in mind: Happy power-levelling!