You have forgot something ^^

Before the fight, make sur that the two of your team members are dead. So the Ruby weapon will put himself a part of his body in the sand. Then reborn your two team members and use some tazers (you can find this object by killing a certain monster in the Shinra Underwater Plane [the place where you can have Hades]). So use your tazers to petrify the weapon, then use the Knights of Round with the mime Materia. Use a tazer again when the weapon can move again and repeat the operation.

The weapon will finaly die