rubywalt

Really, it's ideal to attack Ruby weapon first. Ruby is the one in the desert under the Gold Saucer, that small red blob.
Just go up to it in the highwind or a Gold Chocobo, but with a Gold Chocobo if you want to go in style.
Ruby is the easier out of the two, although some people strangely say that he isn't, so killing him first and getting the AP from his death to up your materia level is important.

First give Cloud the Mystile Armour, Enhance Sword, and a Ribbon. Give him HP absorb linked to Knights of the Round, Mime,
Quadra magic linked to Bahamut Neo/Zero or Hades,
W-Summon and finally Magic Plus and MP Turbo.
Fill all your remaining slots with as many HP plus' and MP plus' as possible
(preferably with a high level) and Final Attack linked to Phoenix. You won't have to use final attack, but the AP from Ruby's death will prove useful for Emerald...
After all this Cloud should ideally have an HP level of 7000+, an MP level of 600+ and a magic level of 200+. You should really have around these levels, or you are going to find it much harder.
Now give another party member Enemy Skills and Sprint shoes,
so at the beginning of the battle that person can cast Big Guard before anyone else,
which will put Cloud in haste which is very useful.

Save your game near the Gold Saucer, and walk into the Red Tendril with your Gold Chocobo.
Immediately cast Big Guard at the beginning of the fight, and hope that Cloud isn't the one who gets Whirlsanded away, if he does,
just restart the game and load from the saved position earlier.
Note: I read somewhere that you can go into the battle with two of your party members already dead from a previous battle, leaving Cloud.
Therefore, this wouldn't let Ruby kill your last member (i.e. Cloud) as he can only kill 2 party members with the whirlsand, and leave 1 left. This tactic has been proven to work.
As soon as you have cast Big Guard, have Cloud cast W-Summon Bahamut ZERO twice at Ruby. Do not cast Knights of the Round yet. Bahamut ZERO will run through five times (eight if Quadra magic is at level 2). You may or may not do damage to Ruby, that is not important,
what is important is that you get Ruby to put hill tentacles
into he sand and behind you. Now you have effectively won the battle. Now cast W-Summon Knights of the Round,
the first at the tentacles and the second at Ruby. KOR wont be cast twice if it's only at level one, but what's important is that you kill the tentacles, which you'll do because you have a extremely high magic level. Now just mime every time; it'll cost zero MP and you'll be getting healed at the same time so it can't be bad.
It should take 4 or 5 rounds of KOR until Ruby is dead. Nice.