Unless my memory is mistaken (and it has been noted) I think you win the Final Attack Materia during the "special" battle at the battle arena (where your last opponent is the Proud Clod.) And you could really use it in your battle against Emerald and Ruby. (Especially Emerald.)
As for beating them. I know one sure strategy against Ruby, but I can only implement it after beating Emerald. I use Hades to "stun" Ruby and then hit him with W-Summon and KOTR. The reason why I wait until after I’ve defeated Emerald is because you can obtain the Master Summon materia from Emerald by returning the artifact you win, so you can endlessly use Hades and KOTR provided you have enough MP (using the other two characters to heal the summonner as required.)
Emerald. A lot of the time it’s been luck, but I’ve beaten him twice, normally by equipping Final Attack – Phoenix for Aire Tam Storm, and then hitting him with KOTR and then miming it constantly using the Mime materia. There are probably easier ways of doing it, but I’ll leave that to others to tell you, because I don’t know them…
Of course, you should put as much materia as possible on your 3x and 2x weapons for leveling purposes. But just be careful – definately dont’ put 9 pieces. 8 is too high too IMO – I’d advise a max of 4 or 5.
– PTM
But I can’t beat Ruby, damn him!
Then Ruby was a bit weird cos once I hurt him with KOR with his torso up!?! must been a bug. The best way it to kill your other two players before you enter the battle then he puts his hands in the sand…and doesn’t use whirlsand on them. after he commits himself just revive them both… but somethimes he brings he claws back up so do KOR quick as possible… but he often uses ultima so I’d recommend Final-attack with revive.