There are many ways to defeat Emerald Weapon and many are very time consuming.
I highly recommend to use the "cheats" in the game.
I used a team with Barret, Cid and Cloud all with level 4 Limit brakes.
Barrets 4th limit can you get in Old Corel after the train sequence later in the game.
Cids 4th limit can you get in the sunken Gelnika ship also later in the game.
Clouds 4th limit can be super time consuming. You have to battle in the battle square inside the Gold Saucer "on disc 3" and get 32.000 battle points to buy his limit. And here is the point what makes this also time consuming. You need 10GP (Gold Saucer points) to battle. For each battle you can get around 3000-10.000 Battle Points. And if fail a battle for this fight you get 0. There are many ways to obtain GP. The Easiest is to "buy" them. Sometimes there is a guy on this spot at the Gold Saucer.
If not just leave and come back again. Here you can buy up to 100GP with 10.000Gil. I highly recommend to rack up 1000GP for 100 battles.
After you have enough GP you can go battle. The battle mechanics are more ore minor complex but what we do we use the "cheats". I used any weapon and any armor (no big differences) but what i recommend is a Ribbon as an accessory.
Up to this point you can have up to two ribbons.
One in the Temple of the Ancients: You reach a room with a giant Clock. Leave the Clock at 5 there is the ribbon.
The second one is in Gaea's Cliff: Here is a video for this ribbon
(Video Credit goes to LetsPlayGGs)
Sadly both are missable. If you miss both Ribbons you can Morph Master Tonberry in the Northern Crater into Ribbons.
In the battle square you have to fight up to 8 enemies. For each consecutive win you get more BP (battle points). The more the higher (
). From round 3000 up to 10.000. That means 4 to 11 battles if you never lose.
But here is what it makes time consuming. After every battle you have a slot mechanik with a debuff. Broken Weapon, Broken Armor, Broken Accessory, Broken Materia of any or all colors, Lvl Down, HP/MP Down and so on. If your accessory is broken its often, without better gear and Materia, impossible to win. Just play again and again up to you 32.000 Battle points. Buy the Omnislash and you are done.
After all this preparations, buy some HP+ Materia,
but a maximum of 6 Materia in total for each character, in Cosmo Canion, to get 9999HP on each character. Also reach level 75+ in the north crater.
And then follow this video.
After the battle you obtain the Earth Harp item. This can be traded in Kalm to one Master Materia of all three types, Command, Magic and Summon.
Take them to get also.