Where to train?

I think Secret Bases might be useful. I've heard of someone who has a base with trainers all with Blissey/Audino (can't remember :c).

 

Basically what's been listed:

Rebattle the E4
Blissey bases to the max
Rebattling trainers when they're ready
Mirage Island audinos
and really if all else fails just train gradually in Victory Road

OR/AS strangely doesn't have high level wild areas, but I think it's just staying true to the original R/S

 

I mostly train by rebattling existing trainers, including the ones that challenge you daily in the Battle Resort. Does seem a bit tedious that there aren't many high-leveled wild Pokémon areas but I've grown somewhat used to it with how many hours I poured into R/S/E. :[

 

Combine those places or E4 with lucky eggs

If you don't know how to find them here you have a detailed info:
Lucky Eggs can be held by Pokémon to increase the experience points they earn at the end of battle. In Pokémon Omega Ruby and Alpha Sapphire, they can only be obtained from wild Pelipper.
Wild Pelipper can be easily found while surfing or walking in tall grass on various routes and cities. The areas they appear on are listed below:

Pelipper's Location

Spoiler

Route 103
Route 104
Route 105
Route 106
Route 107
Route 108
Route 109
Route 110
Route 115
Route 118
Route 118
Route 119
Route 121
Route 121
Route 122
Route 123
Route 124
Route 125
Route 126
Route 127
Route 128
Route 129
Route 130
Route 132
Route 134
Sea Mauville
Slateport City
Lilycove City
Mossdeep City
Aqua Hideout
Magma Hideout
Pacifidlog Town
Ever Grande City
Battle Resort

DexNav's Method

Spoiler

After capturing one Pelipper, you'll be able to search for more using the DexNav's detector mode. Simply tap on Pelipper's icon while using the DexNav to search for hidden Pelipper. Remember to sneak around slowly to enter battle with one.
Each time you battle a Pelipper, your DexNav's search level will increase for that species of Pokémon. You do not have to capture the Pokémon, but it is said to increase the chances of finding rare Egg Moves, Hidden Abilities, better stats, and better items. Increasing the DexNav search level to four will allow you to see if a Pelipper is holding a Lucky Egg before you even battle it.

Compound Eyes Method

Spoiler

You can increase the chances of Pelipper holding an item by placing a Pokémon with Compound Eyes in the lead of your party. Nincada will always have Compound Eyes as its regular ability. They can easily be found on Route 116.

Thief Method

Spoiler

Once you do enter battle with a Pelipper holding a Lucky Egg, use a Pokémon that knows Thief to snatch it from them, or simply capture the Wild Pelipper if you don't have a Pokémon with Thief. TM46 Thief can be obtained from a Team Aqua/Team Magma Grunt in the Slateport City Oceanic Museum. The thieving Pokémon must not be holding an item before stealing the Lucky Egg. If they are holding an item, they will not be able to take the Lucky Egg. Just assure to do it like a Ninja and Run from them without being noticed