My favorite method is to stat and stone them all as mages, and raise them one by one alongside my glass cannons. Mage stones are much cheaper than Rally and Eliminate, and the build is pretty forgiving too. Once you have them all at level 25, you can save up for the expensive stones at your leisure--or not at all, depending on whether you want to actually use them, or just want them to be max level. And you'll reach your goal of a mage from every flight in no time! (Good luck with Arcane and Earth. I've made all my flight reps level 25 mages, and those ones made me cry.)