So very much like cracking Ziodyne super early in DS2.Well, I decided on a whim to see how far I could get in Naoya's route without updating my level 30-40 demons and rushing through to try to one-shot bosses as quickly as possible, and the answer is that with some shameless suspend abuse you can beat the whole game that way. Who would have thought that my two little Mothmen would have such a crucial role to play in demolishing all the Bels. The ending seems to be designed to get you to play through the eighth day content too (was it that way in the original version?) so I guess I'll keep going.
I think it's on Day 2 or 3 when you have to save Fumi and she is possessed by Botis. Botis attacks but his Abraxas only attack and all Botis does is spam Shield All. He has Ziodyne, but will Shield All until he runs out of MP. Just keep guarding until he runs out, then kill him for easy Ziodyne.