Let one guy have the underwater materia because the battle will probably take mare than 20 minutes.That person will have no part in the actual combat. Then equip Cloud, give him the final attack linked with Phoenix, Mime, Sprint Shoes, Magic plus (mastered), and fill the remaining slots with HP and MP plus'. For the last guy, quadra magic linked with Bahamut ZERO, HP absorb with KOR (Knights of the Round), WSummon, Sprint Shoes, the other magic plus, and MP and HP plus'. Now to fight the battle: Start by casting WSummon with Bahamut ZERO and KOR, right after have Cloud mime it. This is the start of a combo, and if you did it right, bahamaut should have been cat 4 times, and KOR once.After just keep miming with Cloud and skip everyone elses turn. After a while the Emerald weapon will try to kill you using Aire Tam Storm, and then Phoenix will revive your group. You'll have to start the combo again, and you should be able to kill him after a while. NOTE: if your KOR is at level 2, cast that instead of Bahamut ZERO, the battle will go faster.