Nitori has to hit a weakness or slayer to deal damage, but then she's golden. But... that would require knowing what you're gonna be fighting :S
Lithos has her 9999 damage last spell at lv95, I suppose. But, well, 2 bombs and 99 mp >_>; There's also, uh, her 6666 damage mystic spell maybe? Apart from that, there's Wild Stab and Pegasus Meteor. If you use Byakuren's elemental sword and combine it with Lithos' Rainbow Wings ability from her Magic Tree, that'll boost her damage by 50% even on an enemy -NOT- weak to fire/water/earth/elec, and makes her awesome if they are.
As for Byakuren, you could consider giving her a staff sometimes to try and use the MP Absorption Staff ability (Never tried it, I know it's ridiculous when the enemy uses it though, so it may be quite nice >_>;)
Endgame you probably have the POW for Reimu to be a good healer -and- damage dealer at the same time, and with her Gohei mp drain, you can even get her to drain significant amounts of mp on physical hits to keep her going. Sanae has this going for her too, if her attacks can actually damage the enemies in there enough to get her mp back; she certainly should have the free turns to do it with every now and then, I imagine. (Sanae's poison on her final weapon is so strong, you might consider trying to get her to inflict it. I don't know how realistic of a goal that is, though.)
Also... 8 rooms? The wiki says it goes up to U. That would be... 21 rooms :V If you don't have all the ultimate weapons (at least the ones that you'd ever want to use, I know some are eh), at least everyone's non-lv99-moves, and some totally kickass gear (Oh man the 15000 youkai kill gear is so amazing if that's realistic; I know I didn't even have 10k kills when I beat the expansion boss, so it was NOT realistic for me. Regalia and Hakurei Ribbon...), then you probably need to prepare more. It's not nice.
Also, as for IND; it does boost buffs, but the amount varies by buff, and overall, Sanae gets little out of it, and Byakuren doesn't get a lot more. Satori gets a good bit out of it on hers, but Kabuff +1 is the only one she's got, and it barely pushes her ahead of Sanae's (I -think- hers becomes better, at least...?) The fact that it stands up to Sanae's at all is great, though, and can make Satori a possible Sanae replacement in parties you were planning to use both Sanae and Byakuren, since Satori also has heals and protective spells. Of course, it'd be more just to speed things up then anything else, by letting you replace Sanae with someone who can deal more damage.