Once you have the Tiny Bronco, Cloud can now acess Mideel by going south of Fort Condor. In this area, there are creatures called Head Hunters that look like little green bugs. If you have the "Beta" ability learned from the Midgar Zolom, you can wipe them all out at once [assuming your around level 35-40]. This will result in an epic amount of EXP and AP points as well as several thousand gil per pop. Utilizing the Rune Armelets you can buy at the Icicle Area Bone Village [double grouth], you can make your "ALL" materia spawn i no time at all. A fully mastered "ALL" materia can sell for 1400000 gil apiece! If your caracters get worn down, just hop on the plane and skip to rest at Fort Condor [saving you money on restoration]. if you stay the course, you can become a Lvl 70-80 with well over 5 million gil! and the best part is, because of materias spawning ability, you dont loose any of your hard earned materia. Also, you can master a lot of other lesser materia like the elemental and fire and ice and earth and such. Also, you can steal X-potions from the white, worm like spinners.
Happy Level Hunting eoff!