Attacks

Sting
MP Sting
Shadow Flare

Stratagy:

Here are some recommended materia and materia combonations for this battle:
Enemy Skill (Big Guard)
Knights of the Round with MP Turbo
Final Attack with Phoenix
Lots of HP Plus
Hades
Mime

Before you battle Ruby Weapon, a wise stratagy is to kill off two of your characters. This will prompt Ruby to immediatly sink its Tentacles into the sand when the match begins. Then cast Phoenix to revive them. Follow that up with Hades on the Tentacles to slow them down. Then cast Big Guard. At this point, Ruby will not have taken these attacks lightly, a Megalixir will fix you right up. Be warned that Ruby Weapon can easilly kill your entire party if you aren't careful. Each turn you should leave one character open to commands in case the need to heal or revive becomes nessacary. That having been said, attack with your Knights of the Round/ MP Turbo combo. Then have another character Mime it. after each round with KOTR, Ruby will counter with Ultima. Big Guard will help lessen the blow, or it can miss some characters completely at times. Have the third character (the one left idle) use a Megalixir after the second KOTR. Repeat this process, re-casting Big Guard as nessacary. Ruby Weapon will fall after about seven or eight rounds.