Beat nightmare Gigases, no CM.
Setup for the L250 fight:
Onion Knight (R3 Full Charge, R3 Fullbreak), Mako Might, FF6 Blood Sword
Rinoa (R1 Meltdown, R5 Valefor), Ace Striker
Shitbrat (R3 Biogrenade, R4 Hi-Unfaith), Dr. Mog's
Selphie (R3 Faith, R4 Maraise), Battleforged
Y'shtola (R5 Wrath, R5 Curaja), Dualcast White
SG RW
Trash rounds:
The trash in this one felt SIGNIFICANTLY more durable than in previous L250 nightmares, but this is partly due to a tweak to my usual setup (I usually bring Krile for her BSSB commands instead of Rinoa, whose BSB doesn't have an MT attack burst), and partly because I was better prepared to fight several enemies at once instead of one tanky enemy, and there were several of the latter. A few SLs here and there until Y'shtola got enough SB for her BSB, then things went more smoothly (quick-hit Curaja is so, so, so, so, so good).
The Gigases:
General strategy went, deal AoE damage as much as possible while focusing down one gigas at a time, ideally only ever having one below ~50-60% HP at a time. With this setup, teardown order doesn't really matter as long as you're focusing them down one at a time with AoE thrown in as efficient. I set up SG before taking down the last round of trash, though honestly I probably should have saved it to have as a panic button early on, or applied it as the first gigas approached its weak phase. Anyway, with that in mind my timings ended up being 1. Refresh SG after taking down the first Gigas and 2. make sure the 2nd gigas is down before the second SG wears off.
Onion Knight did his thing. Once I ran out of Full Charge charges, I switched him to using his Magic BSB command, partly because it outdamages the physical one when not coupled with Full Break even BEFORE taking into account that after faith I am at +90% MAG. I really only ever used Full Break on one Gigas at a time, since I needed the DPS too badly to knock them down in time. I'm also lucky enough to have his AoE Damage + 50% MHP Heal SSB, which gets Powerchained with his Magic BSB command.
Selphie: Shit goin' down? If y, heal. If n, OK/Rinoa buffed? If y, throw out some damage. If n, buff. Oh my god Selphie is slowly becoming my favorite PC, and someday I want to see just how many magic buffs I can get stacked on one character just for fun.
Every Gigas is weak to Earth, so Rinoa's BSB and OSB shined here. Just made sure to build enough SB gauge to always remain in burst mode, and throw out OSBs when I have excess. This ended up being about 4 OSBs thrown out with a ton of BSBs too. The SB gauge build bonus from her always hitting weakness was icing on an already overpowered cake. It's a damn shame she doesn't have Support 4, because access to Wrath would make her better than my Terra FFS.
You can figure out what Shitbrat and Y'shtola were up to during the fight. A couple things I would tweak if I could: make OK go full-mage mode with Chain Bioga and some other 5* BM, but BM is literally the only skillset I do not have the motes to unlock. >_<. This would free up Tyro's Bio Grenade slot (for the mastery reqs) for Break Fever. Still, a minor change that was mostly irritating because it highlighted my OK's sole deficiency in a really obnoxious way. Even though he does have the stats to pull off this dual ATK-MAG build it is still outright weaker than just specializing in MAG or ATK.
So, I noticed that in the final Shitbrat's Collection banner, Rinoa's Enearth dress is a freebie option. After this Gigas fight, I am sorely tempted to go for it.