Tried this a bit and came to the same conclusion as everyone else so far, it's not beatable without discovering some hidden mechanic that nobody has observed yet.
A few observations we made:
- One tank can tank both of them, but will take a lot of damage. For RUN, parrying helps a lot and Battuta gives great reprieves from incoming damage. PLD seems the better choice (Burtgang+Aegis, or possibly Duban if upgraded enough) as they can self heal more reliably due to MP recovery from their JSE, although they also took more spike damage including one time when they took over 2k from a single attack. A RUN that can Magic Fruit might be similarly effective, but as a RUN/SCH even with Tenebrae runes popping pulse every 60s, I was constantly out of MP from curing myself and from having my MP taken. That said, I wasn't using RUN Empy body so that may provide more adequate MP recovery compared to Adamantite, and regardless with a little healing support from the group I was able to survive just fine on RUN/SCH so I think both options may be viable
if they can hold hate (we didn't fight the full 60 mins, just lots of little 10-15 minute test runs).
- Both tanks take a lot of damage and are constantly dispelled to the point that rebuffing is mostly pointless beyond maybe Phalanx and/or Cocoon.
- Chaos seems to really like Swooping Assault, a 20' AOE which does almost 1k to an Epeo RUN, even more to a Burtgang PLD, and can one-shot everyone else. I suspect this move alone takes melee setups out of the running, nevermind the dispelga spam.
- Several dispel moves are greater than 20' AOE. There are also several 20' AOE damaging moves, including some that I believe are centered on the tank. For positioning we had our tank take the wyrms to the side of the staircase in the back of the arena and wedge sideways in there with their back against the stairway. This allowed someone to heal the tank while being 20+ from both of the wyrms
and the tank which allowed them to avoid all moves except the 25' dispels. The ranged attackers were able to hit the wyrms from 25' away and avoid all moves including dispels. It's possible for a GEO to stand within healing range of the tank at about 20.5, be out of range of everything except dispels, and still be within range that their Indi bubble will hit the ranged attackers that are standing further back at 25. (Bubble range is normally 6 so it requires careful positioning for everyone.) This positioning worked well for us because the tank does sometimes get terrored/petrified/etc and need some help from a healer. We generally had the BRD/WHM doing this duty while the GEO stood back by the ranged attackers to avoid losing their Indi bubble, then when BRD had to re-sing, the GEO would move up carefully and cover healing until the BRD was done. Between the BRD and GEO, one of the two always had to be ready to heal the tank in a pinch.
- Chaos definitely takes significantly increased damage from prime weapons which isn't surprising. It also seemed like Bahamut took reduced damage from prime weapons. This included things like nukes, stage 5 Opashoro nuked significantly harder than R30 Bunzi's Rod on Chaos, and seemed weaker than Bunzi's on Bahamut. We didn't test this extensively so I'm not super confident on this point, we were more focused on trying to find an elemental weakness and failed at that as well. Magic damage in general just doesn't seem like the way to go. Kaustra did land for 19k on Chaos without Rayke/Gambit/etc so I wouldn't totally rule this out, but it didn't look feasible. If there
is an elemental weakness, it's probably a shifting one that doesn't stay the same for very long.
- Speaking of shifting, the wyrms seem to have modes, possibly tied to element. I once got petrified 4 times in about 60 seconds, and outside of that I think I was only petrified one other time in all our attempts. They also seem to go through bursts of casting lots of nukes and going hard, then calming down for a time. It could be this fight requires responding to their behavior in specific ways.
- Debuffs are all very hard to stick the first time, let alone subsequent casts, and Chaos was observed removing debuffs from himself fairly quickly. Keeping Distract up didn't seem feasible.
- Separating them with 2 tanks had no noticeable effect on the fight from what we could see.