The only thing you can do for Giga Flare is have high MYS affinity. Level MYS affinity in the characters you usually leave out (in my run it was Meiling, Yukari, Minoriko, Iku, and Komachi as backup) with skp and raise it the rest of the way with equips. I had 300+ on all of them. By now you should have an Ultimate ZUN hat or two, a Karen Device, Arturos Gem, Black Hole Ocarina, even Magic Sword Chaos. Make use of em.
UNC is tremendously strong because unlike Hell's Tomakak it has excellent piercing. Great Hakurei Barrier makes Hell's Tomakak hit pretty soft - but UNC still hurts lots. High FIR affinity is again needed to deal with it, just high MND with buffs won't do in most cases, because Okuu's MAG is so high.
It's important to go into the fight with very high levels in those affinities - and you'll really need to level them later anyway, so might as well do so now. An AoE healer also helps. It's good to see you have Reimu, would be great if you can buff her MAG.
What level are you fighting her at, for the record? Depending on what she casts, you can beat her rather early, but if you don't get lucky she can screw you over while you're high level anyway. I'd recommend 275 as a minimum - I beat her not only after Yuuka and Agastobrauma, but even after Kedamagrammaton. It might look like overdoing it a bit (or a lot), but she can really murder you if the RNG decides to when you're lower. Even so, I'm sure she's beatable, so long as you are willing to spend loads on FIR/MYS.
As for me, I'm fighting the grind wall at the moment. I've hit Meiling 510. The plan is to get in at least 10 levels per day. And it seems like Rhododendron Dress is the only drop that likes me - I have a grand total of five now, three of which were from 30F.
EDIT: Don't forget the only status effect you need to worry about for this battle is PAR. She also does MND down so if you can get some statdrop resistance that's also good, but with Reimu's GHB around it shouldn't be a huge concern. Focus on other things first.
