Half of veteran-GAF has Marie up at the moment. She makes the trial trivial outside of dealing with that pesky and buggy Megaflare.
You need at least one healer with 400 SPR and 5000 HP (Refia or Ysh; bring both preferably) to survive Megaflare. Everyone else needs 250 SPR buffed with focus (so at least 225ish) and 5000 HP to survive while defending.
After the opening round, Love You All trivializes the whole fight until the final threshold (40%), where Bahamut may or may not bug out for the remainder of the fight. At 40% he constantly tries to charge up for Megaflare. But if he bugs out, you are home free.
Unless you chaining Orlandeaus, do NOT bring light weapons. Anything else will work just fine.
Go SLOW over the 50% threshold. If you hit the 50 and 40% threshold in the same turn, you are going to have a BAD time and probably wipe (he 99% gravities at the 50% threshold and if you trigger the threshold after that on the same turn....).
*** Running plausible theory on how Bahamut may be bugging out from reddit (as Nazta datamined his AI):
-- Final threshold (40%)
Begin charging Megaflare
3 random elemental AoEs
After that,
-- If Turn is multiple of 3, begin charging Megaflare
The issue may be IF the following turn after crossing the 40% threshold, which automatically triggers a Megaflare charge is a turn number that would cause a new Megaflare charge (instead of firing off the first Megaflare), Bahamut may get stuck in an infinite loop and never resolve the first or any subsequent Megaflares afterwards.
Coding FAIL, FTW (if true).
Anecdotally, this logic coincides with my run's observations. Bahamut charged up on consecutive turns upon crossing 40% and never fired off another Megaflare for the rest of the fight after that, which enabled my embarrassing run to actually be successful when it shouldn't have.