Who do you run in p3? If the team does not include either Sanae or Byakuren (or both), then you're doing it wrong.
I had my setup a few posts earlier, but it IS a pretty huge text wall. My last party is Alice in front of poison shield, with Reimu/Byakuren/Youmu/Marisa.
Reimu is a LOT more nifty then I originally though, ESPECIALLY considering I just found out her magic barrier will stop Evil Spirits 3-5, which is the only major problem in that phase. Switching between that and Little Legion, with some sequence problems due to needing Legion on nuking turns, should be able to let me handle it now.
I almost won, SO CLOSE, but taking off that Headshot Goggles IV to put a High Ribbon on Youmu lead to a realization that her lv60 last word actually IS capable of missing. Oops. Still had Never Fail on Byakuren and got super close, but didn't end up making it by the tiniest little bit...
I'll probably try again later. I have it so the second phase is about impossible to mess up (And goes without much trouble with Satori/Reisen getting Reisen up to 10k noncard hits and 22ishk Lunatic Red Eyes hits in addition to Sanae's Poison) and the first phase would require a LOT of bad luck to not be able to beat. And with this new revelation, the third phase should be similarly in the bag. It was nice to find out that Cirno's Ice Weapon skills actually -are- better then her magic.