Well, KOTR is actually what you want to stay away from with Ruby. Every time you cast it he'll counter with Ultima. It sucks. The best way to start things off is to make sure two of your characters are dead when you enter the battle. This way Ruby will bury his arms immediately and won't remove one of your characters from the battle. Hades works extremely well against Ruby so make sure you do that. Keep fighting with that character until he dies.

Make sure that they are equipped with the Final Attack-Phoenix materia combo. Then all three of your characters are revived. What you should do with the other two characters is put a 100% Cover materia on them. And then equip like 14 mastered counters. That way, every time Ruby attacks someone, one of them covers and then counters 14 times! That's the best way to take him down. But you have to be a pretty high level to. Somewhere in the 80's or 90's I'd say.