The best way to beat Ruby is with one character preferbly Cloud.

If you have 2 or 3 character alive, he'll knock a random character out of the fight untill there is one and if your best character is gone, you more or less buggered.

In my previous post i put a link to the battle i capture againts Ruby.

When Ruby put his claws/tenticles into the ground, they cause more damage with its normal attack than what damage Ruby actually does.