Boss!Tenshi is like that mid game battle that all the drama has been built up for, think of magus from chrono trigger. And you are more limited item/party member wise, i imagine they couldnt make it too hard so you could win with a limited roster
Meanwhile magatama and shield are the last enemies standing before ame no murakumo himself, you have most of the roster and broken items by that time, NOW they can put any bullcrap abilities on those. at least thats how i see it
Regal was talking about the Mirror and Magatama on floor 12, not the ones on floor 20.
And Magic Transfer is fine. Pop in, do whatever healing / buffing you need, Magic Transfer, get the tank to switch her out. For me, Komachi's not doing a whole lot aside from occasionally trying to land Narrow Confines of Avici / casting emergency heals and neither is Byakuren when everyone's mostly buffed up so I tend to have a lot of people ready for switching.
Sure, you can use multi target DEF/MND buffers but the fact of the matter is, some characters take turns faster than others, thus buffs run out faster for them. Having good single target buffers is still handy. As it stands though, Renko is doing all the buffing I need at the beginning of the fight.
Komachi, Byakuren, Aya, Renko
Aya casts Divine Grandson's Advent (henceforth referred to as DGA) on Renko
Renko uses Charge
Aya generally gets another turn before the boss takes their turn, thanks to the SPD buff from Renko and how I've built Aya so she casts DGA on Renko again for a second round of Charge. This bit is even more consistent if Raid Maneuver from Strategist procs on anyone other than Aya.
If Komachi or Byakuren gets it, Aya still casts DGA on Renko, but instead of relying on Aya getting a second turn before the boss, they can swap Renko out and back in for another Charge cast. If Renko gets it, GG Aya uses her SPD buff to buff herself, Renko casts Charge, Aya casts DGA on Renko, Byakuren swaps Renko out, Komachi swaps someone in to receive god buffs (because my Byakuren is faster than Komachi)
Everyone is now capped out in buffs and Byakuren can start handing out god buffs instantly. I now slowly swap in all my characters as needed to pass the god buffs around before pummeling the boss.