Just like it says on the tin: Get any Materia to MASTER rank! It's almost impossible not to master at least a few materia throughout the game, but if for some reason you're going for this achievement alone, here are some tips:
Easiest done with some of the earliest materias you get in the game. Using a weapon or armor that has double growth rate(or triple, if you've gotten that far into the game without maxing out any materia) to increase the speed of leveling the materia up.
The following materia are easily gotten relatively early in the game, and has incredibly low AP costs for mastering, making them suited for the job:
* Seal (Upper Junon)
* Mystify (Cosmo Canyon)
* Transform (North Corel, Cosmo Canyon)
* Chocobo Lure(Chocobo Ranch)
Cloud starts the game with Lightning and Ice materia already equipped, and Red XIII has Fire equipped when he joins the party. Although higher than the ones listed above, the cost for mastering these are still quite low, and the fact that you have them from so early on makes them also good candidates, as they'll have gained at least a bit AP already when the materia mentioned in the list becomes available.
Now just keep on fighting with the materia equipped until they reach that MASTER rank, and the achievement will unlock!