Here is a list of stuff you need to beat the ruby weapon
TIP: FIGHT THE RUBY WEAPON BEFORE THE EMERALD Equipping everyone:equip Cloud with the Mystile armor, Ultimate weapon, and Ribbon. Equip him with HP absorb linked with Knights of the round materia, Magic plus, WSummon, Mime, Quadra magic linked with Bahamut ZERO, and final attack linked with Phoenix. Use all the other slots with HP and MP plus'. You are only going to use Cloud in this battle. Equip another character with E-skill with Big Guard on it. Give him the sprint shoes also. The last member has no use. Just have him attack or use him to heal everybody. Fighting the battle: Start off by casting Big Guard, this will help Cloud a lot. Then have Cloud cast Bahamut ZERO with WSummon. This may not do any damage, but just provoke the ruby weapon. It doesn't matter if the other members are sucked down as long as Cloud is alive. Wait until there are tentacles behind you, then use Knights of the round on them, and it should kill them since you have a high magic status. You will get healed fully because of HP absorb also. Now just mime all the time and you will get to use Knights of the round again and again. Keep doing that until he dies, and you will get a dessert rose for beating him.
