I use to face the elite four multiple times until my Pokemon actually grew a bit... It took a few weeks to be honest probably not what you'd want to hear but it does work. I had in Pokemon yellow my raichu vs elite four player no1.. I ko'd elite four no# with just first raichu which was lv 52 only.. Try areas like
Victory road use repel for low level Pokemon, go to end of cave so you got a Pokemon centre and an exit nearby
Cycling road, Pokemon here were lv 30
Also The Elite Four. Even if you cant beat them, they yield good exp.
Aaaaand they give a good amount of money out plsu when you fail second elite four you return back to the Pokemon centre and have to face no 1 again and I dont mean to annoy you here but the elite four are nothing compared to the champion, so goodluck you'll meed Pokemon at lv 55 really.