Ok I've developed a huge tactic for defeating Ruby weapon. Heres the stuff you ned to get this right
Master summon equipped to Cluod (trust me)
Knights of the round equipped on somebody else
Master Command on Cloud
A ribbon on everyone,
Shield casted on Cloud
Cloud with Full cure materia
After Ruby makes your other allies sink into the sand cast shield on Cloud and if Ruby casts ultima use Full-Cure if he does it again use mimic, repeat the prosses untill Ruby will give up on casting Ultima, now comes the hard part. Cast Knights of the round. After this your sheild may fade off and Ruby will case Ultima (maybe) cast full cure, repeat this prosess at total of 10 times (for me that is) and Ruby will die. GOODNIGHT EVRYBODY.