Here is the part two of my tips! :
Okay, according to my info, the ruby weapon should be left for the last. I have a strategy here, but I'm not sure it'll work. But it worked for me, though. Okay, here goes:
First, work your brains out to get the gold chocobo using the old method (the part where you train your chocobos to rank S), and go get the knights of the round Materia. After that, un-equip all your materias except for the Kights of the round, Phoenix, double-slash, W-item and mimic materia. Go fight the dumb emerald weapon. (In the sea). Notice when his life reaches to 3/4 or less, he starts to use the Aire-tam Storm and does damage equal to how many materias you equip. so, of you equiped 4 materias, you'll be dealt 4444 damage. So think wisely and don't equip too much of them. Fighting Emerald Weapon. Start off with your double or 4X-slashes. When he starts to change and 4 stupid round objects on his body appears, use your knights of the round to wipe them out. Repeat process and frequently heal. whenever you are dealt damage, HEAL! even the damage of a 100 can mean one-shot kills. Eventually, I took almost an hour to finish him off. And so, good luck trying my tactic!