When I beat Ruby the Hades Summon was my best friend. It will give even the powerful weapons stop, silence, and a few other things.
As always start with two characters dead so he'll bury his claws, the W-Summon Hades and Phoenix to revive everyone. You should get another turn before his stop wears off, so you can set up your combo to finish him. I did W-Summon Hades (to make sure he always stays stopped) then Knights of the Round, then Mime until he's dead. I beat him this way without taking a single point of damage.
edit: I just read lestatdark's comment above about not using KoTR on Ruby, but I never had him counter me with Ultima, does he have to wake up first, then use it? Also, having the entire party revived would be bad news if he had woken up, but I wanted the experience and the AP for everybody and their materia.
