Freeze as the orbs move out, move in, and shotgun Marisa. Then, -before- the orbs start moving back in, go towards one of the lower corners (left might be easier), freeze, then slip into the corner. Keep shooting to restore your charge, then freeze as the orbs start moving in. You should have room to move under Marisa and shoot her from the bottom of the screen. Just repeat that as necessary. Pretty trivial when done properly.
With the exception of first round Shikieki, if I'm not mistaken, who is pretty much the same as Lunatic Match Mode AI. So I think with characters like Reisen or Mystia it's pretty much impossible to no death 1cc D:
Actually, I'd be very surprised if that were the case. She's perfectly beatable on the lower difficulties, so I suspect that with Lunatic it's just that she lasts an obscenely long time before letting herself get hit. Someone like K.B. might know for certain.
I might also say that if she really were unbeatable, and you were somehow able to survive (which is relatively doable as Reisen, for instance), you'd have pretty much unlimited scoring potential, and that alone might be a reason to doubt that she's designed to last forever at max rank. Assuming ZUN cared about the scoring enough, anyway.