Reaper 1 turn kill (without Theurgy)

Skip to 1:12 for the fight; I just thought Akihiko tweaking was hilarious enough to be left in.

Party members needed are (placed in ascending turn order):
1. Koro/Mitsuru
2. MC (Heaven's Blade build with all boosters, with Shift Amp/Boost accessories)
3. Junpei (Slash Amp/Boost accessories)
4. Akihiko (Strike Amp/Boost accessories)

Koro casts Debilitate or Mitsuru casts Rakunda. No distinction here apart from Koro's battle characteristic which grants Auto-Masukunda. (Only Rakunda is important; I just used Mitsuru because I can.)
MC uses Heaven's Blade to get an almost guaranteed crit (~98% without Revolution) on the Reaper. If crit, repeat.
Junpei uses Brave Blade, Akihiko uses God's Hand. If any of those moves crit, shift to MC and cast HB.

I don't exactly remember the odds, but IIRC the only way to fail this 1 turn is if no one crits in a turn. It relies of your MC's build quite a bit. Cups - Charge isn't required for this as well, as the intention here was to create a reliable strat for B2B Reaper farming with guaranteed resources. Many thanks to CTOBN for helping to refine this.

*Left out Single-Target Boost on Cendrillon by mistake, but functionally there's no difference.

1:12 - Fight