There are many ways to level but there is a simpler way to level that i have in store for you.

For Pokemon Lv.10-Below

If you have any Pokemon Lv.10 or below, then dont train in wild Pokes train on the Trainer called Kiddy and Bugtrainer. You train on Kiddy until your Pokemon are lv.6 or higher, some need to be Lv.7 or higher. Kiddy has both a Level.3 Ratata and Pigey. This takes less then an hour. Once you have your Pokemon(s) in the correct level start training on Bugtrainer, he is harder then Kiddy. He has both a Level.5 Caterpie and Weedle. This may take between 1 hour to 1 hour and 30 minutes.