I've found that the best way to grind Sprite's Magic is to find an area close to an Inn. It doesn't matter what kind of XP you get from the mobs, just keep opening your spell menu and casting on all enemies until you run out of magic. Then stay at the Inn, and repeat.
Also, using the sprite debuffs and lowest MP cost spells are the better ones to use since the xp required for a level is dependant on how many spells you cast.