Hm... I'll try to help. I was a little overpowered at that time and had no real problem with him, but I can guess what's wrong. I know E. and T.Pts. are not much in Cathedral Ship, but as I remember Exp is okay, I think somewhere around level 16-18 you could pull it off.
I found chaos and KOSMOS do well against him, and I think Ziggy would do just as well as Kosy, but unfortunately I find him too slow
If you used MOMO or him, you can try putting her behind Ziggy and abusing Bodyguard. The boss can't push him down easily, but Ether attacks are a pain until the two underlings go down. Never put MOMO in the front row. No HP = quick death. She's not much against the boss, best to preserve her for healing.
Shion is essential, as she has Medica All, transfer this to the other two if you can, it saves life, but I rarely had her attack. Best to put her in the back and have her guard/replenish her EP/heal with Medica All/Steal from the boss (you might even give her a boost from an item so she can cut in the line in emergency if the other two can't heal), her HP and Def isn't very good and after the lackies are gone, the phys down attack can do a number on her. Don't let HP go below 50% for anyone, best to keep it around 80%.
KOSMOS has a good amount of HP, Def/Edef and generally well around stats. chaos comes close second, less HP but also good stats. Both are good in the Ether department, so they can use Medica/All with efficiency.
Basically what you need is slashing and thunder attacks, such as upgraded R-Blade, Heaven's Wrath, or even Shion's Lightning/Shock Blade. Try putting them in Hi-Slot, that way you can execute them every turn. If one character accumulated boost, use it in emergency only.
I hope it helps,
~Vejitto