I'd suggest training around Eterna City. Routes 205, 206 and 2011 are you best places imo with pokemon ranging from 14 - 19 in levels. Really, the lower the level of the pokemon you wish to train, the further toward the beginning you should train. I'd suggest training from the very frst route, and spend a fair amount of time on each grinding. Basically, you're playing the game all over but instead grinding each route until your pokemon are high enough to move onto the next. It may take a fair bit of time, but in the end it'd be worth it!
Also, I'd suggest you find yourself an exp. share. That way you'll be bale ot share exp. earned in battle by other pokemon with other members of your team; incredibly useful for those in situations like yours!