In the International, North American and European versions of Final Fantasy VII; Ruby Weapon, is one of the two secret optional or additional bosses in Final Fantasy VII. Unlike Emerald Weapon, there are no prequsities or time limits to fighting it. It's located right by Gold Saucer and you'll need to encounter it using Highwind. Distinct from Emerald Weapon, Ruby uses status ailments to affect your preformance in battle as well as ejecting your party members.

General Recomended Materia
+ HP / MP Plus
+ Final Attack Linked To Full-Life or Phoenix
+ Mimic
+ Knights of the Round
+ W-Summon
+ Hades
+ Enemy Skill
+ Quadra Magic

First, you've got to protect yourself from any status changes, and increase your resistance and defense (Ruby Weapon loves casting Ultima) with Enemy Skill's Big Guard and equip the Ribbon accessory. Next, if you'd like to keep your best character in, make sure you either kill off your characters whom you don't like or you just have them down before you start the battle - it's a gamble whether you should wait it and and try. Ruby will use the ability Whirlsand to eject your characters, so watch out. Also, everytime you attack physically, Ruby will counterattack with Ultima. Cast Knights of the Round with W-Summon and then keep using Mimic as long as you can. Basically keep yourself at normal and keep damaging with maximum damage.