Once you get Dryad, you will have all seven elementals for each spellcaster. I suggest you use the least expensive spell and just cast it over and over. Many elementals have 1 MP spells that will make the leveling quick and easy. The higher the level you need, the more times you have to cast a spell to get to the next one. It works out to be 9 spell XP toward the next level less the current level of that elemental. For example, at level 0 you get all 9 points so you need to cast 12 times to get to level 1. From level 7 to 8, however, you only get 2 points per cast so you need to cast 50 spells. As a bonus, if you max out your level 8 by casting 100 spells then the visual effects change and the spells do even more damage.