---Equiping Your Team---
Equip Cloud with the Mystile armor, Enhance Sword or Ultimate Weapon, and Ribbon. Equip him with the HP Absorb linked with Knights of the Round, Magic Plus (make sure you have about 220-250 magic power), W-Summon, Mime, Quadra Magic linked with Bahamut ZERO, and Final Attack linked with Phoenix (this is just a precaution). Use whatever slots you have left with HP and MP Plus'. Cloud is going to be the only character you are going to use. Next, equip another member of your party with the Sprint Shoes and the Enemy Skill. The last member is going to have no use in this battle. You might want to have Turbo Ethers or Elixirs around just in case.
Fighting the Battle:
Start off the battle by having the person with the Big Guard Enemy Skill cast Big Guard. Make sure you do this very fast and before anything or anybody has a chance to do anything. This will help Cloud a lot. Make sure that you save the game before the battle because the Ruby Weapon might suck Cloud in, but don't worry, just keep doing the battle until Cloud isn't sucked down. Right after Big Guard is cast have Cloud cast Bahamut ZERO with W-summon. This may or may not do any damage, its just to provoke Ruby Weapon to stick is tentacles out behind you. It does not matter if you other members get sucked down, just so long as Cloud is alive. When you see his tentacles behind you cast W-Summon Knights of the Round (if your running low on MP use an Elixir or Turbo Ether.) at the tentacles, (it wont be cast two times if its at level one but it doesn't matter, the main thing is killing the tentacles) this will kill them thanks to the high magic power you have due to the Magic Plus materia. Plus you will get healed because of the HP Absorb. Now just mimic every turn and Knights of the Round will be cast twice even though it could only be cast once before! And because you are mimicking it won't cost any MP! Just keep doing this each turn and you will eventually kill him. He might try to use Comet2 and other spells but thanks to the Mystile armor it will usually miss. When you beat him you get the Dessert Rose.
Equipping your team:
First pick the guy who has got the highest magic and HP level (Cloud), next equip a loser/cannon fodder with the under water materia. Now equip Cloud with Final Attack linked to Phoenix, Mime, Sprint Shoes, Magic Plus, MP Plus and the rest HP plus materia. Clouds HP should be over 8500.
For the last character equip Quadra magic linked with Bahamut ZERO, HP Absorb linked with Knights of Round, W-Summon, Sprint shoes, the other Magic Plus and enough MP plus' to get the MP over 800.
Just before you battle, save your game. In the battle, make your last character cast W-Summon once for Bahamut ZERO and once for Knights of the round. After, have Cloud mime it. You will see Bahamut cast 4 times and KOR once. If your magic levels are high enough, this should do 150,000 points of damage. Now wait for Clouds turn again, skipping everyone else, then have him mime again. Everyone else will probably die, but keep on miming with Cloud. You will probably be able to mime three more times because you would have been healed to full health every time (HP Absorb). However, by the fifth time Emerald gets hit with KOR, he will get annoyed and unleash his Aire Tam Storm. You will die, but Phoenix will resurrect you. Put your last character back into action by casting W-Summon Knights of the round on Emerald. Mime again with Cloud once that is finished.
By now you've hit emerald eight odd times with KOR, so one more mime from Cloud should be enough. Even if you die, Phoenix will resurrect you all and you can cast W-Summon Bahamut ZERO once more. Then Emerald dies and you get loads of EXP and AP and can feel deeply satisfied that the it is dead.