Just got back into this game, and after getting Reimu up to level 450, I gave the Serpent of Chaos another attempt. I think I'm getting closer: everything went really well until halfway through the second phase, when Iku got sniped by a fourth-slot Steel Slasher. With no offensive buffs for Exorcising Border I lost Yukari to an overdose of physical attacks, and then Meiling got a Thousand-Handed Kannon to the face right at the start of the third phase. And finally, to end the whole trainwreck, a Magical Hellfire got through Reimu's 28 PAR resistance, leading to her just kind of sitting there alone tanking magic for a whole four turns before she went down. :V
On another yet related note: Iku has got to be one of the most useful people in my party right now. She's probably indirectly the cause of about a third of the damage my nukers do thanks to her amazing buff, and there's no way I could have gotten this far with my only healer being Reimu (and Meiling, though she hardly counts) without it.
Lastly, why won't the 30F randoms drop any equipment for me D: