Ruby Weapon
Most people say it's better to attack Ruby first. But I say he is still pretty tuff. Ruby is the one in the desert under the Gold Saucer, that small red blob moving around. Just go up to it in the highwind or a gold chocobo, but with a Gold Chocobo if you want to go in style. Even though you must only fight Ruby with only one character, he is much easier than Emerald who can do 9999 damage to all people and heal himself in his terrifying Aire Tam Storm. Ouch. First give Cloud the Mistyle
Armour, Enhanced Sword, and a Ribbon. Give him HP absorb linked to Knights of the Round Table, Mime, Quadra Magic linked to Baumat neo/zero or Hades, W-Summon and finally Magic Plus and MP Turbo. Fill all your remaining slots with as many HP plus' and MP plus' as possible (preferably with a high level) and Final Attack linked to Phoenix. You won't have to use final attack, but the AP from Ruby's death will prove useful for Emerald... After all this Cloud should ideally have an HP level of 7000+, an MP level of 600+ and
a magic level of 200+. You should really have around these levels, or you are going to find it much harder. Now give another party member Enemy Skills and Sprint shoes, so at the beginning of the battle that person can cast Big Guard before anyone else, which will put Cloud in haste which is very useful. Save your game near the Gold Saucer, and walk into the Red Tendril with your Gold Chocobo. Immediately cast Big Guard at the beginning of the fight, and hope that Cloud isn't the one who gets Whirlsanded away, if he does, just restart the game and load from the saved position earlier. Note: There has been a rumor going around that if you enter the battle with two of your characters dead Ruby can't kill Cloud because his attack allows him only two kill two members.
As soon as you have cast Big Guard, have Cloud cast W-Summon Bahamut ZERO twice at Ruby. Do not cast Knights of the Round yet. Bahamut ZERO will run through five times (eight if Quadra magic is at level 2). You may or may not do damage to Ruby, that is not important, what is important is that you get Ruby to put hill tentacles into he sand and behind you. Now you have effectively won the battle. Now cast W-Summon Knights of the Round, the first at the tentacles and the second at Ruby. KOR wont be cast twice if it's only at level one, but what's important is that you kill the tentacles, which you'll do because you have an extremely high magic level. Now just mime every time; it'll cost zero MP and you'll be getting healed at the same time so it can't be bad. It should take 4 or 5 rounds of KOR until Ruby is dead. Nice.
Emerald Weapon
Man this guy is the toughest of them all trust me. With about 1,000,000 HP it's going to take a while as well. When he is annoyed, he unleashes his Aire Tam Storm which does 9999 unblockable damage to all party members, and heals himself. Which is pretty annoying after you kept healing yourself.
The requirements for defeating him are as follows:
1 Final Attack at least level two.
2 Phoenix at the same level as Final Attack.
3 Knights of the Round at level 2.
4 HP Absorb.
5 Quadra Magic at level two.
6 Bahamut ZERO at level two.
7 Two magic plus materia (one at master level).
8 Several HP and MP plus materia at high levels.
9 One or two Mime materias (two preferred).
10 Two Sprint shoes.
11 W-Summon.
The battle is going to take at least an hour to complete, so getting the underwater materia from the Man at Kalm is very important. He'll only give it to you if you morph a ghost ship into a guidebook for him.
First pick the guy who has got the highest magic and HP level (Cloud), next equip a loser/cannon fodder with the under water materia. Now equip Cloud with Final Attack linked to Phoenix, Mime, Sprint Shoes, Magic Plus, MP Plus and the rest HP plus materia. Clouds HP should be over 8500.
For the last character equip Quadra magic linked with Bahamut ZERO, HP Absorb linked with Knights of Round, W-Summon, Sprint shoes, the other Magic Plus and enough MP plus' to get the MP over 800.Just before you battle, save your game. In the battle, make your last character cast W-Summon once for Bahamut ZERO and once for Knights of the round. After, have Cloud mime it. You will see Bahamut cast 4 times and KOR once. If your magic levels are high enough, this should do 150,000 points of damage. Now wait for Clouds turn again, skipping everyone else, then have him mime again. Everyone else will probably die, but keep on miming with Cloud. You will probably be able to mime three more times because you would have been healed to full health every time (HP Absorb). However, by the fifth time Emerald gets hit with KOR, he will get annoyed and unleash his Aire Tam Storm. You will die, but Phoenix will resurrect you. Put your last character back into action by casting W-Summon Knights of the round on Emerald. Mime again with Cloud once that is finished.
By now you've hit emerald eight odd times with KOR, so one more mime from Cloud should be enough. Even if you die, Phoenix will resurrect you all and you can cast W-Summon Bahamut ZERO once more. Now Woopie hes dead or should be dead.
Diamond Weapon
This is by far the easiest weapon to beat. He is the one who comes out of the water and heads toward Midgar. All you need to do is lay the smackdown by casting magic and summoning GF's. Remember that he is invulnerable to physical attacks until after he does Diamond Flash. Anyway, cure yourself when you're low on HP and that's how to beat Weapon. You can also steal a Rising Sun from him.
Ultima Weapon
This is by far the most annoying weapon. He keeps running away and you spend most of the time chasing after him. It sucks when you loose him. The first time you fight him, all you have to do is survive his attacks and attack back for a couple of rounds. The next time you fight him, I forget but here's how to beat him. When you can, take the Highwind over Lake Junon (waterfall, deep pool, near Junon). If you see a large black monster over it, that's Ultima Weapon. If not, try again later. When you fight him the first time, any attacks that are long range will hit him. When he's taken enough damage, he will fly away. Now, you have to chase him in the Highwind until he stops moving. Then you'll have to fight him again. When he makes his final stop at Cosmo Canyon, beat him and he will explode in the canyon creating passage for the ancient forest. You can get the Ultima Weapon after you beat him.