In the original English version of FF VI, devise is used for Sabin's Blitz ( "Devised a new Blitz" ). "Mastered" may work, only if you think that something that does not already exist can be mastered, and is created as it is mastered. ( personally I'm fine with devised )
For the plural of the word materia ( Latin and Italian word for matter, and likely other languages too ), I just want to point out that in Latin the plural form is materiae, and in Italian materie. Now you may want to treat materia as it was an English word or a fantasy word to begin with ( why not ) and then materias would do. But when pronouncing it, isn't the final sound "-as" too short, too abrupt ? Whereas something like "-aes" (materiaes) would sound longer when pronounced, more airy... would be better ?