General Tips And Info
First, some beginner's tips:
Now, for the advanced tips:
1. Final Attack + Revive = When the character that has this combination dies, he/she will cast revive and bring him/herself back from the dead.
2. Elemental + Most spells = Protection from that magic when equiped on the armor slots, and adds that effect to the attack when attached to the weapon slots
3. HP Absorb + Deathblow = Regain HP while dealing critical hits
4. Cover(mastered is preferred) + Counter attack, Counter attack, Counter attack, Count..... = The character that has these equiped will cover any attack that the enemy hits you with and then counter a good many times.
5. Mega All + Steal, Mug, Deathblow, Manipulate, Sense = Many neat combinations. Mega All is my favorite materia because of the combinations that you can have with it. Such as having mega all, deathblow + HP absorb. Hit all enemies with deathblow and gain hp while doing it
6. Quadra Magic + Ultima or Comet 2, W-magic = Cast the spell 8 times. Nuff said
7. Mp Turbo + Knights of Round = Knights of the round will do much more damage, over the course of 13 knights, this will add up tremendously
8. Mime + Limit Break = This only works when there is only one character fighting, such as the Ruby Weapon or battle arena. With Cloud's omnislash, you can just mime it over and over and over racking up 15 hits at a time for about 5000-7000 a hit.
Red Weapon (In the Gold Saucer Desert):
If you wish to have all 3 people when fighting the boss, just follow these simple steps. First, choose one person to have either the revive-all combination or the phoenix materia. Now, have that person kill of the other 2 characters in another battle besides the weapon one. You should now have one character with the revive-all etc.. combination. Equip everyone else with materia following the paragraph below's tips, and head into the weapon. Since you have only 1 character, it will almost always place it's arms underground at the beginning of the battle. Now, cast life 2 all or phoenix and revive everyone, and there you are!
First off, equip everyone with the necessities, such as barrier + all, time + all, and restore. If you have full cure(cosmo canyon, back of shop), equip that too. Equip your characters with the summon materia Bahamut ZERO(Cosmo Canyon, Huge materia) and Knights of the round(NE island). With the person who has Knights of the Round, equip with mime and a mystile if you have it. With the person who has Bahamut ZERO, equip a W-Summon.(battle arena, 64,000 BP)Equip any other characters with mystile as well, because this will give them a 60% chance of avoiding all of the weapons magic attacks, even ultima. Give HP pluses to those who need it, to try to get in the hp range of 7000-9999. equip any other materia that you think will help (shield <last crater>comes to mind). Also, equip as many people as you can with the final attack + revive materia. Now, fly into the weapon, and it will begin the attack. Your time meters start out as nothing, so to counter-act this, equip 2 people with sprint shoes. Equip the 3rd with a ribbon(Clock scene in ancient temple). If you don't have the ribbon, equip him with a sprint shoe as well. As soon as you start off, cast regen, wall, and haste. While you are casting these, the weapon is probably attacking you. Cast fullcure on those who need it, then just concentrate on keeping everyone's hp up and setting up defenses. Cast shield if you wish, because this will prevent anyone from taking damage from Big Swing. Attacking him at this point is useless, since his arms are protecting him from taking any damage. Just hold off for a little longer. Now, he may or may not have cast whirlsand, and if he did, he cast it twice. Reducing your party to one. If you were lucky, he did not take the character with the Bahamut ZERO or Knights of the Round. Keep holding off you attack until he throws his arms into the ground. Now is the time to attack. If you have Knights of the round, cast it, but be prepared, because he will counterattack with ultima. If you dodge the ultima, then he will attack with a ruby ray. This doesn't do too much to a person with wall on, about 1800 damage. If you are at a good amount of hp, then do the command MIME. This way, you can unlimitedly cast that summon, as long as you cast it the last round. Beware of the two spikes behind you however, they cannot kill you, but they reduce your HP very low, leaving you succeptible to other attacks by the weapon. If you have the hades materia (sunken shinra plane) equipped, cast it on the two spikes behind you. This will stop them and poison them, leaving you only with the weapon itself. Continue casting knights of the round and curing and casting wall. If he gets off a lucky hit, you should have final attack and revive to save you. If you have them both mastered, you should have no problem killing him. For undergoing this long ordeal, you get a desert rose. This can be trading in at Kalm, on the second floor of the house in the far right. The guy there will give you a Gold Chocobo.... Not too much of a prize eh? But you can brag about it to other people :)
Green Weapon (underwater):
This weapon I consider to be less of a threat than the Ruby Weapon. It still has about 500,000 hit points. Just be sure to read the part about the underwater materia.
Follow the same materia equiping strategy as listed above. There is one special materia that could come in helpful here however, called the underwater materia. This eliminates the 20:00 counter while fighting the weapon. The way to get this materia is to fight a monster called the ghost ship in the underwater reactor. Fight along the tunnels with all the little fishies outside. When you come across the ship, morph it into the Guide Book. Take it to the guy at Kalm in the second floor of the house on the far right, and he will give you the underwater materia. Now, it helps to have each of your characters HP at 8500-9999, but final attack + revive or phoenix (Condor Tower) can be substituded. Many of the weapon's attacks deal plenty of damage, in fact, it has an attack that deals 9999 to all of your characters. Since you have all 3 characters for the whole fight, he shouldn't be too hard. Try to cast a w-summon knights of the round, and then have someone mime it. Having 2 or 3 people miming it can help this battle end more quickly. Be sure you have wall, haste and regen cast on your characters, or else you won't stand a chance. For killing him, you get an Earth Harp. Give it to the guy at Kalm, and he will give you the master summon, master magic, and the master command materias!
AN ALTERNATE METHOD
Hey! I just found a(relatively) easy way to defeat Emerald Weapon! You need to have Knights of the Round and Mime Materia for this to work, though.What I did was to get Cloud down to about 6277 HP, then used 3 hi-potions on him out of battle. When I came into battle with Emerald Weapon, Cloud had All Lucky 7's and hit Emerald Weapon nonstop 64 times for 7777 hp each hit (this takes off nearly 500000 hp). Then Emerald Weapon used Emerald soot, and I had Cloud cast Knights of the Round with W-summon, which took off about another 220000 hp. The next step was to have Tifa Mime Cloud's W-summon, for yet another 220000. This brings Emerald Weapon down to just about 60000 hp. After this, I had Tifa Mime herself once more, and the fifth Knights of the Round finished him off. I spent hours trying this and that until I came up with the idea to enter battle with All Lucky 7's. Without it I would have been mashed yet again by that outrageously powerful American secret boss.