Try investing in some crafted accessories. Doesn't have to be the i110 one, two i90 will max vitality melds will suffice. You'll get more HP without sacrificing too much mind then.
In general you can do without Divine Seal or Presence of Mind here. But you could use Divine Seal to conserve MP. For example: For flatten -> flare breaths you could spam Cure, rather than Cure II to save a bit of MP.
In addition, I'd suggest trying to feel out when earthshakers are about to come out. During earthshaker cast and effect, Bahamut does not deal any damage to the main tank. Try and get a stoneskin up on the main tank right as the cast goes out. Bahamut does a flare breath + auto attack right after the earthshakers. This is optional, though. But it'll definitely make your tank feel safer (they can dip surprisingly low from this).
If the transition always happens after megaflare/before earthshakers, have everyone stack up after megaflare. Use Divine Seal for Medica and Cure III here to top everyone off. Depending on what the scholar does, Cure III may be enough. It's safer in general to get everyone completely topped off, though. Even if that means overhealing. Don't use Medica II for the third megaflare if possible. Competent tanks could get the add off you even with Medica II up. It does prevent any potential add-jojo'ing, though.
As for post-gigaflare. While Gigaflare animation goes off, pre-cast a Cure III while your party is stacked up. Try to time this so it heals up the part before the add appears. Talk to the scholar who heals which tank and stoneskin yours right after. Virus the first and third add too if you want to and ask a caster to virus the second. Apply the virus right as it appears, the first thing the add will do is an auto attack -> Deadly Drive combo. Once the add's hate has been established, park yourself at the center of the area and Swiftcast Medica II. It should hit nearly everyone and top them off before Megaflares.
When Megaflare goes off: Run towards the center of the area, use Divine Seal and time Medica in a manner so it heals up the marked group right after megaflare hits but still heals them before any potential pillar megaflare goes off. The marked group can also get a pillar, it's always under -someone-. Two things to do now:
If someone has two stacks: Cast a stoneskin on that person after the Medica, otherwise idle for a bit to conserve some MP.
If no one has two stacks or is in no immediate danger of dying: Have the entire party stack up for Cure III before rage. Don't pre-cast this so it heals up the rage afterwards. The add appears before the damage is dealt and you -will- build enmity on it if you do.
After that Cure III toss a Regen on both tanks while your Divine Seal is still up.
You could stoneskin the tank before megaflare goes off if it helps them. Otherwise you have your hands full healing the entire party already. While it -is- possible for you to keep everyone alive before rage (main tank included), it seriously eats away at your MP if you do. Plus, you're not solo healing here. Let the scholar do their share.
If you use Medica (II) -> Cure III in this situation, everyone should be topped off anyway. You could decide to stoneskin them "just in case", though. But you only have time to get one stoneskin out between Medica (II) -> Cure III.
To summarise:
Phase 1 is pretty solid. Maybe stoneskin the tank before earthshakers, but that's it. For the third megaflare don't use Medica II. Use Divine Seal Medica instead and have the party group up for Cure III after
Phase 2's flow is like this:
Giga -> Pre-cast Cure III -> Stoneskin your designated tank -> Virus add -> Regen one tank -> Top off your own tank -> Regen other tank -> Swift Cast Medica II from the center -> Stoneskin the tank for Flatten -> Megaflare -> Divine Seal -> Medica -> stack up -> Cure III -> Rage of Bahamut -> Regen both tanks while Divine Seal is up -> repeat from "virus add" (have a caster virus the second add)