Emerald Weapon is the hardest boss in the entire game (fortunately, you don't have to fight him). It was added in so the game would present more of a challenge for us westerners, and a challenge it is. With roughly 800,000 - 850,000 HP and the ability to deal 9999 damage to everyone in your party simultaneously, it isn't something to be taken lightly.

Location
Emerald Weapon can be found swimming around in the area accessible by the submarine. If you don't want to accidentally get in a fight with him, stay as high as you can. This usually keeps you too high for the Weapon to come in contact with you. When you do get in a fight with it, you'll know because the submarine will ricochet backwards in a flash of light (like when you try to enter the Crater in Disc 2).


Preparation
Don't even THINK about trying to beat the Emerald Weapon until you have maxed out the levels of at least one person in your party. It will utterly destroy your team if you aren't very powerful. If you want some Ethers to help replenish MP, you'll get them for free when you fight around Mideel. I snatched up around 70 from the local beasties after battles. If you have W-item it may be useful to make copies of turbo ethers etc if you have high mp levels... Collect the following items as early as you can, to aid in Materia mastery
Equipment
Equip everyone with their ultimate weapon if your Materia is mastered or you don't care about Materia EXP, or else the second best. Your levels should be in the 90's if you think you're going to take on Emerald Weapon, so forget the "EXP Plus". Put the "KOR" and "HP Absorb" in linked slots on the person with the highest MP level. Equip all with "Mime", and "Revive" and "Final Attack" in linked slots. If you don't have the preferred amount of a materia, you can tediously fight around Mideel until they're mastered and a new one is born, or you can improvise with what you have (my choice). Mastery of "KOR" and "Final Attack" is essential, because each can only be utilized as many times as its current level, with a maximum of five times. If you take the time to get a Master Summons, you can cast KOR unlimited times as long as your MP allows. Ziedrichs can't be bought, so if you've already defeated the Turks without stealing one you can't get one. In that case, use your best armor. The Ziedrich has no materia slots, unfortunately. If you do have materia troubles, replace the Ziedrich. Don't add a "Barrier" unless you really feel like it's necessary, because it is removed upon death. "Restore" probably isn't necessary either if you have enough X-Potions and Elixers. You could put an "HP<->MP" on your KOR attacker so he wouldn't run out of MP, but he'll be running out of HP every other second and "Final Attack" is only used five times. If you want to cast the KOR as often as possible, you can put a "Quadra Magic" on the "KOR" instead of "HP Absorb" to cast it 8 times per turn per character with the X-Summon. Keep in mind, though, this means no quick and easy HP regeneration. "Counter" isn't essential, but it's those little things that give you the edge on the Weapons. One or two extra KOR mimes can mean the difference between winning and losing.


Battle Strategy
When the battle starts, just start up the KOR chain and keep it going as long as possible. He normally dies after about 10-12 castings, so you'll have to mime at least five times unless you have the Master Summons. Don't bother curing anyone if they are hurt but not dead, because they'll probably be dead before you have someone ready to heal them. If they do last long enough, they'll be able to cast or mime KOR and gain all the health they need off the "HP Absorb". Once you get Emerald Weapon mad, he doesn't whirl you away like Ruby does. Instead, he opens up his four regenerating mini-lasers, two of which deal thousands of damage and two more which drain 300-500 MP. Emerald then immediately attacks the entire party. Fortunately, the lasers are rather weak (relatively speaking); one blast of KOR knocks them out. They only got one shot off at me before they died. Then, they were regenerated later to be destroyed again. (We managed to defeat Emerald weapon in 8 summons of it... on W-summon and mimed 4 times, we also had hp absorb on the first caster of K.O.R so that each summoner after that lost no hp after the summoning, if Emerald attacked)


Post-Battle
After you defeat the Emerald Weapon, you get 50,000 of each in EXP, AP, and Gil. You also receive an Earth Harp. Do NOT go thinking that Earth Harp is just another stupid prize! if you exchange THIS one at Kalm and you get EVERY MASTER MATERIA! All the summons, all the spells, almost all the commands! But hey, after draining 800,000 HP without dying (permanently), it's well deserved!!
Emerald Weapon
3x "Final Attack" From: Battle Arena Comment: MUST HAVE!
3x "Revive" OR 3x Pheonix (you'll have to master pheonix about 2 times for this though..) From: Most Materia shops Comments: Pair with "Final Attack"
1x "X-Summon" From: Battle Arena Comments: More KOR's
1x "HP Absorb" From: Mideel shop Comments: Pair with "KOR"
1x "KOR" From: Round Island Comments: Need Gold Chocobo to get this summon! 
3x "Mime" From: Cave east of Mideel Comments:Need Blue Chocobo
3x "Counter" From: Crater Comments: Pair with "Mime"
1x "Underwater" From: Kalm Traveler Comments:Eliminates time limit
3x "HP Plus" From: Mideel shop Comments: To get all characters HP up
1x "MP Plus" From: Mideel shop Comments:To allow more KOR's
1x Gold Chocobo From: Chocobo Farm Comments:To get Materia
1x Level 99 From: Mideel area Comments:(best EXP) "EXP Plus" helps
3x Ziedrich From: Steal from Rude Comments: Best armor
Emerald Weapon
Beat The Weapons MainPage
Hosted by www.Geocities.ws

1