So I'm guessing you are not supposed to be able to beat Miss Darkness with Drain, or at least not yet? Like, progress was slow, but after I did some probably excessive grinding, and understood more of how exactly her multi-drain worked, I did manage to do it, by having everyone with a Time Watch, starting the battle with the bare minimum EXP for Level 100 and killing normally where it was convenient, and having Sam and Vix defending on alternating turns to prevent their own bleed, and only healing Red unless one of them got below 3000. It definitely helped that my grinding got my Drain Boost to an extra 810%, but I imagine if you defeat the all the Boss Souls without draining, with maybe 20m EXP on each character (just enough to deal with the Kitsune without having anyone dip below Level 100, and then you can funnel Miss Darkness's back to everyone), then you don't need the boost to be that high, since it seems your highest character's total EXP determines how much she drains (like I said, alot of grinding. I had Red with 750m and the others with 235m, and she drained way more than I could ever keep up with.) I didn't think to get a picture of the victory screen proper, but I did get one with her right before it, at level 94, so proof the strategy does work.