Emerald WEAPON
The first of the new creatures roams the ocean floor and makes its first appearance when the team acquires the submarine. You'll want to aviod this creature early on, sice your party simply won't have any chance of destroying it. Later acquiring the Final Attack, Knights of the Round, Mime, W-Summon, and Mega All Materias you can put up a decent fight.
So how tough is Emerald WEAPON? Its basic attack does about 7000 points of damage, so it can immediately knock out a character. Once its smaller lasers open, it can hit the party with eight consecutive attacks. Four of the attacks do about 3000 to 4000 points of damage. The other four drain about 300 to 500 MP with each hit. To make matters worse, it can also hit the team with its super weapon that does 9999 points of damage to each character. As far as HP goes, the weapon has well over 500,000 hit points and you've only got twenty minutes to destroy the creature.
First, collect the Underwater Materia (refer to Kalm Traveler section) before you head into this battle. Having it dquipped eliminated the 20 minutes time limit and makes the fight much easier. You'll need a lot of HP and MP to put up a decent fight. Mastered Materia are a must. You'll want to use each of your Materia as much as possible. Try using W-Summon to cast two Knights of the Round and have each of the members of your party equipped with Mime. If they can mime the summons, you can cast an endless chain of Knights of the Round until the creature dies. Also, equip as many characters as possible with the Final Attack-Revive Materia combination. This will help it the creature uses its ultimate attack or just gets a lucky shot. Have lots of Turbo Ether on hand, since Emerald WEAPON can drain your MP quickly. If yo uwin the battle you'll receive the Earth Harp. On its own it's not much, but it can be traded in at Kalm for some incredible prizes (check the Kalm Traveler section).