It'd have involved changes to your party composition, but the has very high DEF but very low MND, which makes it more suitable to attack it with MND targeting/defense ignoring NTR/SPI attacks.
Huh, I did not know/forgot about that. I think I do have Yuuka leveled up, so she'd be a good alternative for Murakumo's Blessing, though I still doubt she'd be as strong as Tenshi, since Tenshi also has the keystones. I'm going to try her out nonetheless.
Also, I have noticed the uselessness of DEF/MND a bit myself, actually, but haven't figured out why that is (and I'd like to find out). This, in turn, makes their equivalent buffs (and probably debuffs, for the most part) useless, too, which is pretty sad, for an element that I believe to be critically important in this game.
But yeah, if the NTR/SPI affinities are the same, I'm also baffled by Nitori's lack of damage output, considering her attack stat with Maintenance (?). I wonder if it was the lack of synergy skills and Overheat (I think it was called?), early on in the battle? Maybe you'll find out something I haven't, if I showed you.
Oh well, time to experiment.