Spirit Blade

Like you, I used Moxia, Terra, Miti, and Aja for stage 1. I had the advantage that I'd leveled every character up to 50 though. Fortunately, aside from Moxia the only character of these you'll need to use after Stage 1 is Aja.

For stage 1, I'd have Aja debuff the demon and Terra buff the party with a defensive boost in order to keep damage levels low. Aja could help with healing if need be (two actions per round thanks to her sword, plus a whole lot of 5,000 healing potions).

A bit slow this way because the demon's not doing enough damage to raise SP points quickly, but it's less risky, and I can melee attack and guard to raise SP some. I tried to end Stage 1 with Aja at full SP, though this isn't really that important, as it's not her SP abilities that are crucial to stage 3.

I didn't worry about whether Moxia would have full SP because she'd gain those quickly enough in stage 2.

Stage 2 ring loadout: Omni for Moxia, Stat for everyone else.

As soon as the stage would start, I'd immediately have Goldi use Omni Raise to fully buff the party (and then have Moxia, Spirette, and Halla use the status block items to make the party immune to status ailments.)

Moxia will get her buffs wiped off fairly quickly (I kind of wish I had 4 stat rings) but the other 3 characters (who are all much squishier than Moxia) are immune to debuffs and so will retain the defensive buffs.

With full physical and magic buffs on the party aside from Moxia, a Recover spell from Goldi and a Recover II spell from Halla can pretty much heal the party completely even after a full series of attacks from the boss. He'll still knock the party down pretty close to death, VERY close to death sometimes, but as long as the Double Physical Defense buffs and Double Magical Defense buffs remain on he shouldn't kill anyone, and with the Double Agility Buffs the healers will go before him.

Fortunately, since he'll be doing so much damage, Goldi will be gaining SP points like mad, and will reach 100 SP before Omni Raise wears off. This is important because if the buffs fall off the party dies very, very quickly.

With Goldi/Halla healing, it was up to Spirette and Moxia to do the damage. Fortunately, Ice works okay on the boss, so Spirette's ice magic and Ice nuke does good damage, and Moxia can hit him with her magic sword attack while charging up her spirit blade attack.

I actually used Spirette's health point check to monitor the boss, and made sure to have both Moxia/Goldi/Halla full on SP when he died. You can guard/heal if he's close to dying and some of the characters are low on SP.

I nearly screwed up at the end and forgot to monitor the status ailment protection, and it dropped off, but fortunately the boss only had a few thousand hitpoints left at this point and I killed him.

For stage 3, I used Moxia, Goldi, Halla, and Aja. Aja is the key, I do not know if it is even possible to beat this stage without her (and without her Flash Katana that lets her act twice in a round.)

I changed the ring loadout a bit: Omni for Aja, and Stat for everyone else. I did this because I was tired of having Moxia's attack buffs stripped off her right when I wanted to do a Spirit Blade. However, your Aja is lower level than mine and thus weaker, so you may want to keep her with a Stat ring so that her defensive buffs can't get removed. Anyways:

Here's how you beat the boss: he's vulnerable to blind and debuffs. As soon as the battle starts, have Aja reduce both his attack and magic attack. Have Goldi fully buff the party with Omni Raise, and use Halla's Neutralize super ability to make the whole party immune to status ailments.

Keep hitting him with the debuffs from Aja, blind him as well and reduce his agility with debuffs. He'll buff himself at various times but you can counteract that by just debuffing him again and again. You'll need to do this regardless actually because the debuffs don't last long, but with Aja's Flash Katana you'll be able to keep the boss's attack and magic attack fully debuffed.

With that, and blind, and party defensive buffs, he'll barely be able to do damage. Frequently he actually did zero damage with his attacks. The only problem is this meant raising SP was annoying, and Goldi couldn't reach 100 before Omni Raise would wear off. So I had to have her use her normal defense and magic defense buff spells. Fortunately, as with the boss blind and its attack and magic attack fully debuffed, I survived the transition.

Damage on the boss basically just came from Moxia. I had her use a fully powered Spirit Blade close to the start, then hit the boss over and over again with Magic Blade, slowly raising SP back to 100, then hit the boss with a second fully powered Spirit Blade, which killed it. So this is a bit of a tedious fight.

Also a very easy one, at least with a lvl 50 Aja. The fight went from nearly impossible to trivial. It's actually incredibly frustrating (as I'm sure I don't need to tell you.) There's no way to know that the boss would be vulnerable to blind and stat debuffs (or more specifically debuffs that reduce it's two attack stats), and so no way to know to take Aja into the fight (and you can't even save right before to experiment.)

Stage 4 is a one on one between Moxia and what's his name. Moxia is restored to full health and SP and magic before the fight, so just using Spirit Blade one shots the boss. At least as long as you have the Omega Sword (it sounds like in the first version of this, if you didn't have the Omega Sword Moxia wouldn't do enough damage to one shot him, and the fight would be basically unwinnable. Joy.)