@Eyelovepie: goldenrod dept store iirc
Backtrack to every route and fight every trainer you didn't beat the first time around. Since I'm usually a completist and must fight every trainer in every area, I usually have overleveled Pokemon. When you gain the ability to rebattle gym leaders, abuse it as much as possible. Since you can rebattle gym leaders as many times as you want at their time/day, they're great sources of EXP.
I too am a completionist, and unfortunately it did not overlevel me at all. When I went into the elite 4, I had Gyarados at 44, Ampharos at 40, Heracross at 38, and an un-leveled Ho-Oh. My Feraligatr was at 37 when I gave up on it after realizing Gyarados was superior in every way, since its stats are way better, it learned every relevant move Fer did besides Crunch, and Dragon Dance seemed to sweep any problems I might encounter in the elite 4 (this was a correct assumption) so I got to spend victory road's wilds + Clair's gym and the 3 battles in the Dragon Den leveling up Gyara to DD.
But yeah, I'm definitely nowhere near even leveled let alone overleveled. =/ Rebattling doesn't occur until all 16 badges are earned and I'm only needing the EXP farming at this time to even myself with the second half of gyms.