Beating the Weapons
So you've completed FF7 up to this part, huh? Well, these 2 battles right here are the only real challenges left and probably the only real challenges in the game itself (aside from Demon Wall), so if you're ready, read on for coolstuffXL's handy dandy guide to beating the Weapons.
First, let's go after Emerald Weapon; contrary to most guides, it's harder beating Ruby Weapon than Emerald Weapon. Sure, EMERALD may have an instant kill attack, but Ruby Weapon can just throw your characters out one by one, and he has a lot of attacks that can almost kill your entire party. Besides, when you fight him you only have one character at the start of the battle (read the beating Ruby Weapon strategy below for more info).
Emerald Weapon has an attack that can kill you in one shot no matter what your life is; he has 1,000,000 HP; and get this, he can regenerate his eyes if you kill them and at the same time attack you around eight times by attacking you physically and draining your MP. So, even if you have Final Attack, if you get hit by this attack you probably won't have enough MP to cast it when you die. So, all in all. Emerald Weapon is a big problem, but don't worry, as he isn't invincible (after all, I killed him). The trick with him to is to do really (and I mean really) fast attacks with your Knights of the Round and keep miming them.
Underwater near Junon. It'll appear only after you've killed Ultima Weapon (the flying dragon, not Ruby Weapon).
First equip Knights of the Round, Final Attack, Mime, W-Summon and Revive, or better yet, Phoenix Materia, to the appropriate characters and then link HP Absorb or MP Absorb (your call) to the Knights of the Round. If you have more than one Knights of the Round then you've got it made. Then, link Phoenix to Final Attack on of your characters or more (if you have more Final Attack and Phoenix Materias). Now equip everyone with their fast growth weapons (don't worry, if your weapon is weak we won't be using the weapon anyway. We're only using Knights of the Round) and put all of the materia you want to grow on the extra slots. Make sure, though, that your stats don't dip so equip MP and HP Plus Materia too.
Beating Emerald Weapon:
Okay, it's the hard part now. Enter the fight and immediately cast Knights of the Round with the character who has W-Summon. Then, as soon as the menu comes up for the next character (he must have the Mime Materia. If he doesn't, then pick defend quickly and wait for the next character with Mime. That's why it's better to have 3 mimes; so that you don't waste precious time) quickly select mime and you will cast two Knights of the Rounds again (speed is the key in this fight; if you keep miming really fast there's a good chance Emerald Weapon won't be able to attack you. In my game, I kept miming so fast that Emerald Weapon had only one turn, and he used that turn to regenerate his eyes. Then, just keep doing mimes real quick, as in, once the menu appears, don't delay, and he should bite the dust in about 10 or 11 Knights of the Rounds. If he does get a chance to attack you though, don't worry, since every time you attack him with Knights of the Round you gain HP because of your HP Absorb Materia so just keep repeating the miming process and don't do anything else because if you do, the chain will be broken (note: when you mime your MP doesn't decrease, so don't worry about your MP). Again, after 10 - 11 Knights of the Rounds he will die.
If you made it this far congrats! Now, keep reading to see want you get after he dies, and oh, by the way, SAVE YOUR GAME!
You will get the Earth Harp item. Now, take the time to save your game (do you want to go through that again?) then go to Kalm and to the top right house on the bottom part of Kalm (note that this isn't a shop; it's a house and it should have stairs). Now, go up the stairs and and talk to the guy. He will say "Oh, the Earth Harp!" and will ask you to trade it with 3 Master Materias (Master Magic, Master Summon, Master Command). Isn't that cool or what! (note: I think you have to talk to him twice. First talk to him and he'll say something about going on a journey and needing 3 things; the Earth Harp is one of them. Then, talk to him a second time and it'll go as mentioned above). Congrats with those Master Materias you now have: all the Magic Materias, all the Summon Materias and almost all the Command Materias all in one materia! Also, you got 500,000 AP * 3, assuming that you used the Apocalypse Sword and other triple or double growth weapons.
Now that you've killed Emerald Weapon it should be easier to kill Ruby Weapon than trying to get Tifa to date Cloud (well, maybe not quite that easy, but easy nonetheless). The important thing with Ruby Weapon is that the only way to hurt him is to have 2 party members dead in the beginning of the battle. Then, as soon as Ruby Weapon puts its tentacles on the ground, revive your party and start with the Knights of the Rounds.
He's a red tentacle you'll see sticking out around the desert of the Golden Saucer. You'll need a Golden Chocobo to reach him. Once you have your Chocobo go to the desert around the Golden Saucer and go near the red tentacle. Ruby Weapon will pop out and the fight will begin. Ruby Weapon will only be there during the later parts of disc 2 and disc 3
You will need the same things you used for Emerald Weapon, but add the Revive Materia linked to all materia (and you should have Life2 by now because you'll need it). Use the Revive Materia linked to all instead of the Phoenix Materia because in this battle you'll definitely need to revive your party with full HP (that's why you need Life2. If you don't have Life2 build up until you get it). Also, add your Master Materias and use the Apocalypse Sword and the other fast growth weapons.
First, equip Revive linked to all materia with Cloud or whichever party member you intend to be the one who revives the party. Then, equip your materias evenly (make sure everyone has the Mime Materia, Knights of the Round and Final Attack). Also equip Mystile Armlet or Gigas Armlet if you have them, as they make Ruby Weapon miss in his Ultima magic attacks. If you have more Revive Materias then equip then with the rest of your characters. Also, don't forget the Master Materias.
Beating Ruby Weapon:
First, before you begin your fight, kill off your characters except for the person who has revive linked to all (let's say it's Cloud). Now enter the fight and as soon as Ruby Weapon puts its tentacles on the ground, revive the rest of your party using Life2 linked to all. Then, cast Knights of the Round with W-Summon and mime and mime and mime away like before against Emerald. Speed is the key, so as soon as the menu shows up select mime really quickly. After about another 10 to 11 Knights of the Rounds Ruby Weapon should also bite the dust. However, be careful with your healing, as Ruby Weapon counters every Knights of the Round with an Ultima spell. If you can't take the Ultima spells, try using Bahamut Zero (which does 1/13th of the damage that Knights of the Round does, so either level up your characters or equip magic defense items). If you attack Ruby Weapon while its tentacles are not in the ground, one of your party members will be eliminated for the battle (they cannot be revived). Congrats again! Now keep reading to see want you'll get for all of your troubles and migraines!
You will get the Desert Rose, which is another one of the items on the Kalm guy's list of things he needs to have to begin his journey. So save your game, and go back to the Kalm guy and talk to him. He'll say this: "Oh, it's the Desert Rose!" (big surprise) and offer you a trade. Accept and you'll get a Golden Chocobo! (so it's not as good as the 3 Master Materias you got when you beat Emerald Weapon, but it's still cool! Congratulations on beating the two hardest bosses in the game!