the last dungeon is the Northern Crater and with Triple growth equipment on the Materia will grow like crazy there, but id suggest for a little boost till then to use any double or triple growth equipment you get your hands on until the next set of better equips come along... also farm the Sunken Gelnika for sources to boost all the characters stats, have your 2x or 3x growth equips on when you do that and youll effectivly kill 2 birds with one stone...