Originally posted by X-Nightcrawler
Christ it must be ridiculously impossible . . .
No, just VERY hard.
taken from the UNAUTHORIZED FFVII Strategy Guide:
"Compared to Ruby, Emmy's a snap. You'll have to use the same basic strategy here, but things are complicated by the fact that Ruby's initial attacks tend to suck away two random party members, leaving but one character alive to face the challenge. Even worse, Ruby Weapon is completely invulnerable in the early part of the battle, and you'll probably have to waste one casting of Knights of the Round on Ruby's arm stalks, lest they suck away all of your MP.
You basically have two options here. One is to load up one character with Knights, W-Summon, Mimic, Final Attack + Revive (which requires less MP than Phoenix, and is more efficient if you only have one charcter), the Shield Materia (it's completely invaluable on this fight), a Ribbon (to prevent the confuse condition Ruby likes to cause), and the Mystile armor (no armor will allow you to survive Ruby's repeated casting of the Ultima spell, but Mystile's high magic evasion rate gives you about a 50/50 chance of dodging it). Fight with Ruby over and over again until that character is the lucky one she allows to live, and then get the combo started. You can start hitting Ruby from the moment the stalks appear, although at that point the stalks themselves should be your #1 priority. Cast Shield on yourself to keep your chosen character alive while you pull off the combo, then cast Knights at the stalks once and at Ruby once, mimic it until your shield gives out, then have a Turbo Ether or Elixir and start it over again. Ruby will kill you repeatedly, but you can stand to die once for each level of your Final Attack materia… More than that, and you know what'll happen. The other option is to just keep fighting that red data animal until she allows two or more characters to stay in the fight, something that happens randomly, and not very often."
By "Combo" they mean have someone use W-Summon to cast Knights twice and then have the others mime that, even the original caster. Honestly, Ruby Weapon was probably the hardest boss in a FF game that I've ever had to defeat (Omega Weapon in FFX didn't even come close) but it definitely wasn't impossible.