Before fighting Ruby Weapon you need good materia.
So it's time to beat Emerald weapon. First go to the underwater reactor passage and look for a monster that is named Ghost ship.
Morph it with Morph materia and you ll' get a item named(i don't remember the name...).
Anyhow go to the Calm village and go to the Calm traveller that he's asking about this item.
Give it to him and you ll' get the Underwater materia that is nessecery
to beat Emerald weapon. Now it's tim to beat him.
Make sure you have at least two Mime materia, one Knights of Round with two stars materia, W-Summon and a MP turbo.
Eguip at your characters Mystile and Aegis Armlet and put them on the rear position, have Ribbon and Tetra Elemental to your characters. Eguip Cloud with Knights of Round and MP turbo in the next slot(Make sure that the slots are together!). Eguip W-summon too on Cloud!
Equip the other two with the Mime materia.( Eguip your characters with HP plus materia to maximize your HP)
Now get on the sub and go to the ocean. Find Emerald weapon and get ready to beat the lod bastard!
Use Big Guard and then summon Knights of Round! Mimic the move with the other two characters!
Don't worry if you see someone dead or with little HP we don't have time for those things!
If you use KOR around 10-12 times you ll' beat him!
After you beat him take the Earth Harp that he will drop and go back to Calm Traveler.
Give it to him and he will giva as change three master materia! Take them and get read for the next fight!
Ruby Weapon
Eguip Master Command to one of your characters, Mime and Knigts of Round to the other and Master Summon to Cloud.
Then eguip them as before on the fight with Emerald.
But nowthere is a tric to beat Ruby. Kill all your characters on your party exept Cloud.
Then have Cloud eguiped with Phoenix combined with Preembtive support materia.
When the battle begins Cloud must use Phoenix. Before that Ruby will burry his hands to the ground.
Then when your other characters will be alive summon with Cloud Knigts of Round and keep on mimic it! Ruby it's must tougher than Emerald but luck is gonna be with your side!
When you gonna beat him take the Desert Rose who will drop and take it to the Calm Traveler.
He will give a Gold Chocobo.