The early stage is really the roughest, later on you'll be able to xp by running roulettes or simply the highest level dungeon you have available, but there are still a few options available to you: First off, you can do levequests. They may have been introduced to you at level 10, but there should be a level 5 camp near town so you can begin doing them then. In addition, FATEs are quite a useful way to level up so keep an eye out for them.