I highly suggest grinding on Victory Road. I assume you have no access to Lucky Eggs, so you are in for a long haul.
It will take about 45 minutes to get a Pokemon from LV 40 to LV 60.
Sorry for the delayed re-response. I had to test out a few areas.
Once you get to level 55 or so, however, you can simply invest all of your money into Full Restores and Revives. then, simply grind on the Elite 4 until you die (don't use the items you bought. You only bought those so you do not lose money when you die).
This method will get your Pokemon to level 60 from 40 in about 20 minutes (each).