This video is an answer to Acro, who made a few days ago a similar challenge with Nanaki's Howling Moon. I'd never have this idea without him, so all credits go to Acro for this discovery. ;-)
Weapon : Death Penalty
Armor : Mystile
Accessory : Jem Ring
Materias : Time - Added Effect (useless here) and HP Plus on weapon ; Poison - Elemental, Transform - Added Effect and Mystify - Added Effect (useless too, cos Vincent is immunized against Confusion, Berserk and Frog when he uses a Limit Break, and I forgot that ^^) on armor.
With this equipment, I'm immunized against all Ruby's bad statuts, but Poison. The most important trick here, is Ruby's left tentacle attack. She inflicts [Poison], but she's not Poison-elemental. So you can be poisoned by the left tentacle, but with Poison - Elemental, you'll absorb damages of the Poison status, as an illimited Regen !
Chaos increases both Defense and Magic Defense to 100%, so if Vitality and Spirit are already at 255, all Ruby's attacks will be ridiculous (31 HP max, for Shadow Flare), except percentage based damages, but they can't kill... And as you'll gain 312 HP for each turn, it's impossible to die, except if you're not poisoned quickly, but it'd be really unlucky. So it's a way easier than Acro's vid with Nanaki and his Howling Moon. I'd say it's even not a challenge... ^^
Entire battle lasts more than 3 hours, cos I didn't increase Death Penalty's damages ! It doesn't really matter, as you won't do any other control than choosing Chaos, until the battle finishes. ^^