Took about 3 hours, but I finally beat IX D???, and with a sub-30 too! Got 29.09.
Team:
Eiko (LM1+LM2): R4 Dark Ifrit, R4 Curada, MM (BSSB), dark/bio resist
Vivi (LM1+LM2): R4 Chain Firaja, R4 Meltdown, DMT (USB2/USB1), dark/bio resist
Kuja (LM2+LMR): R4 Necro Countdown, R2 Hastega, Gathering Storm (OSB), major bio resist + minour dark resist
Zidane (LM2+LMR2): R4 Flash Disaster, R5 Storm Assault, Scholar's boon (USB2), dark/bio resist
Marcus (LM1+LM2): R4 Fire Assault, R1 Mug Bloodlust, Thief DMG+ RM (USB), dark resist
RW: Realm chain
Magicites: Lakshmi (L20 MNd Boon/L10 Dampen Dark), Phoenix (L8 Blade/L8 Spell Ward), Belias (L15 Empower Fire x2), Typhon (L15 Empower Wind x2), Madeen (L20 Mag Boon x2)
+100 magia on everyone but Marcus in their main stat (MAG for Vivi/Kuja, ATK for Zidane, MND for Eiko). Eiko/Vivi/Zidane/Kuja all have various waters too, though I didn't max Vivi/Zidane HP to help them trance more easily.
Rough turn order:
Eiko: Curada to Vivi, Dark Ifrit, magicite, BSSB, Curada Marcus, Curada Vivi, BSSB, Curada Kuja, BSSB or Dark Ifrit
Vivi: USB2, Chain Firaja x3, USB1, Chain Firaja spam* (theoretically)
Kuja: RW, Necro Countdown spam, OSBx2**
Zidane: Flash Disaster x4, USB2, Storm Assault spam
Marcus: Mug Bloodlust, Fire Assault x2, refresh chain, USB1, Fire Assault spam
Some notes:
Hilariously and annoyingly, I couldn't keep Vivi alive after he did his USB1. Cause right at that time was the transition from Phase 2 to Phase 3 - I'd eat Incubus VIral Smoke, then Ultimate Meteor, then Ultimate Flare. Because my party was generally weak after Incubus Viral Smoke, this is when I'd usually be using Relm BSSB. But that stupid 80% HP Ultimate Meteor would put Vivi in a hole, and even with QC, I simply could not get another turn to heal him up before Ultimate Flare got him.
Thankfully, on my winning run, I didn't need him after that. Zidane/Marcus/Kuja closed things out.
Speaking of Kuja, in shades of what I ended up doing with Agrias against Deathgaze, I decided it was better to save his gauge for OSBs for the end of the fight instead of using his BSSB1 in the middle and OSB at the end. On my winning run, I goofed and accidentally did his BSSB instead of OSB a second time, which probably lost me about 40,000 damage. Again, thankfully, looks like I didn't need the extra damage.
I tried Marcus with Fire Assault and Dash & Slash initially. But in my failed attempts, I never got to use Dash & Slash and still had hones on Fire Assault. I switched D&S to Mug Bloodlust to not just give MArcus more bite, but also to reduce some of the incoming damage. It wasn't enough to keep Vivi alive without curing him in P1, but it was just enough to let Kuja tank P1 Ultimate Chain Bioga.
Keeping Kuja alive here is.. not easy. There were times when I was wondering if I should switch to Dire Heal to take the heat off of EIko. But I felt I needed that extra damage throughput. Dunno if it would be easier with Dire Heal, but it doesn't matter now.
That's 13 D???s down, 7 sub-30. Remaining ones are I, III, IV, and XIII.
III I'm waiting on the Jump Start Renewal to pick the III set since that should guarantee a win if not sub-30.
IV I fantastic survivability between Edge SSB2 and Rosa USB1/USB2, but I'm a bit questionable on DPS. Kain USB1 is my only notable DPS, and I figure I might as well wait 6-hit dragoon ability drops before trying this. Otherwise, I only have Tellah BSSB for support, and my last slot will either be Yang (BSSB) or Golbez (USB1), not sure who to use.
My XIII tech is ancient. And pretty bad. Not just Vanille BSSB as healing (which I hear is sufficient for this fight), but I'd be relying on things like Serah SSB2 for support and Ye Old Raines BSSB as primary DPS. Those three can work well together, but I'm not sure who else to bring: Lighting w/ USB1 could probably work, though I don't have her dualcast LMR. I suppose Sahz w/ SSB could work, since he can boost Lightning's damage and blink stuff, but that's pretty ghetto. And XIII D??? is supposed to be one of the harder ones. Not looking forward to this one at all.
I I will probably try last since, aside from Sarah USB1 which I picked up somewhat recently and her BSSB which I've had for ages, I've got nothing good in this realm. I can debuff Garland into the ground but I have no DPS. WoL BSSB/OSB (and recently got his En-Holy LMR, which is nice) and LD Thief w/ SSB are my only real damage dealers. I have Wol USB1, but that won't help with the heavy incoming damage and the defensive debuff is too short to really take advantage of. I have Garland dual-cast LMR and nothing else. When this first released, I couldn't even get out of Phase 1 without running into Dead End. So unless Fest free pulls gives me a bunch of goodies in this realm, this will likely be tackled last.
So it looks like I'll be tackling either IV or XIII next, unless the Jump Start Renewal drops soon, in which case I'll go after III.