In the International, North American and European versions of Final Fantasy VII; Emerald Weapon, is one of the two secret optional or additional bosses in Final Fantasy VII. Emerald Weapon can be found underwater and can only be found using the Submarine. You'll have to get the Underwater Materia (if you don't want to fight under the time limit) from the Kalm Traveler. You'll need to give the Kalm Traveler the Guide Book, located in the Underwater Reactor (Junon). Morph a creature called the Ghost Ship and bring the item back to him and he'll give you the Underwater Materia. There are several strategies to beating him, but overall you'll need to have a continous barrage of hits that deal a the maximum amount of damage, 9,999. If you have a difficult time trying to locate Emerald Weapon, try going to the Crashed Glenika and entering and exiting. Your Materia will also need to be maxed out for growth.

General Recomended Materia
+ HP / MP Plus
+ Final Attack Linked To Full-Life or Phoenix
+ Mimic
+ Knights of the Round
+ W-Summon
+ Quadra Magic

Make sure your characters are all maxed out HP and MP using HP or MP Plus and have the best equipment already equipped. A Final Attack and Full Life or Phoenix Materia in a connected slot means you'll be able to come back after you've died. However you'll need a good amount of MP. Use your prefered spell with Quadra Magic, recomended Ultima or something powerful. Use W-Summon to summon Knights of the Round multiple times. Use Mimic to continue the cycle of continued barrages with attacks. Limit Breaks and Mimic with counter can also be effective. Watch out for Aire Tam Storm, it won't mean instant death - so it's possible for a character to dodge it, but it's 9,999 damage.