I wouldnt say 255 stats are esential, my stats are around 180 - 200 and so long as I have...

Auto-Haste
Auto-Pheonix
Break HP Limit (although it doesnt matter how much hp you have)
Ribbon

He really doesnt pose too great a problem. These armour abilities are essential for your main members.
Ribbon is esential because the only thing that kept finishing me off with Dark Yojimbo was Petrification with an immediate shatter effect (which stoneproof doesnt work well against). I never knew Dark Aeons dropped armour with Ribbon (although it normally doesnt have everything else you need) and killed DArk Valefore, Ifrit, Ixion and Shiva without collecting any so I went round the hard way. I captured all the fiends for 99 Dark Matter, fought One Eye in the monster arena about 200 times for the other 99 and I'm trying to get my last one. I know I can bribe Land Worms for 675,000 for 4 but thats just annoying, anybody have any other suggestions?