I find this battle the hardest out of other dark aeons >_>. I didn't use Zanmoto (cheater!) when I killed all of other dark aeons which is kind of an accomplishment for me in my X career.
The sisters constantly own' my party members by 99,999 damges in every hit. Auto-Protect doesn't seem to work, as well as Auto Life.
I don't have celestial weapons, my accuracy/magic stats both are around 30 (I found them useless, I'm not kind of a magic guy) but I have other stats maxed out - such as Luck, Str, Def and Magic Def.
I've added a Piercing ability to each of my party members' weapons, but I noticed when I attacked one of the sisters, piercing doesn't seem to work (it was supposed to ignore the target's armor), I only can do around 40,000-60,000 damages. Blitz dmg was like only 10,000 each.
This is interesting, I need to think of a new strategy.