That's what I did, was go into the Northern Crater. I mastered every materia I had (which I'm pretty sure was all of 'em), including Summons (damn, that took a while), many more than once. Then, I went to whatever-the-hell-that-place-is (forgot...where Red XIII is from) and got the Master Summon, Master Magic, and Master Command materias...then I went back and got another Master Summon, and two more Master Magic, and whupped the Weapons' asses. (Was there another Master materia?)