I'll post what you don't want to hear first.
Victory Road
I, too, was vastly disappointed by the Pokémon levels in Victory Road. Across the game, actually, wild Pokémon levels were way lower than Trainer Pokémon's levels. However, there are a lot of trainers in Victory Road (even though it's sooooo much shorter than the BW or DPPt Victory Road). You also battle Wally at the end.
Trainer Rematches
Seriously, even though this isn't what you want, this is an awesome way to get Exp. The Trainer's Pokémon get higher and higher levels the more times you fight them, thus yielding more and more Exp.
Secret Bases
Probably the best way to grind Exp is fighting Secret Base Grunts with high level Pokémon. Look around, a lot of people advertise Exp grinding Secret Bases. I saw this one guy (I don't remember where) who's team all had Lv. 100 Blisseys in Rotation Battle format. He did testing while holding Lucky Egg, and each Pokémon that participated gained around 70,000 Exp per Blissey. The best advice I can give you is to search for bases like this.
Aarune
A tedious strategy, but effective nonetheless, is the battle Aarune in the Secret Base Guild. And then battle him again. And again. And again. The first battle will be against his Flygon at Lv. 23. For every battle after that, it will be Lv. 53. This is a long, boring, way of Exp grinding that is hardly time-efficient, but deserves a mention.
Wishful Chaining
Every so often, when chaining Pokémon, you will find one that is 15-20 levels above the rest of the area's wild Pokémon. It doesn't happen very often, however, and is even worse than the Aarune method. If you wish to try this strategy, then do it on Victory Road.
Those are, really, the only way to grind Exp pre-E4 in ORAS. If this doesn't answer your question, then I guess you're out of luck :(
