Hmmm... There's a chance Sayaka might try to copy Riona during the fight.
... We could easily warn her to not do that because Soulguca is perhaps the most dangerous target to try and copy ever.
And after Riona's detained, we can just act first, before Sayaka can even think of copying? I think that works, then.
I also don't want it to be suspicious.
But I think this works.
Added:
[X] Pretend. Visibly restrain yourself. Agree verbally to leave her alone, but keep talking.
[X] At the same time, signal Mami to tag you and Homura and have Homura activate her magic.
-[X]
If Riona is somehow included in timestop:
--[X] Act first: Detain Riona with Grief Fog, take the Gem, threaten to break it.
--[X] If
anything goes wrong, no holds barred. Purée her. Save the Gem unless Riona
still tries to attack.
--[X] Otherwise, continue.
[X] In timestop:
-[X] Consult Mami on the danger of tripping Riona's shades. Explain briefly. Employ Grief hax if needed.
-[X] Leave, convene with Mami and Homura.
--[X] Make sure Riona remains time-stoped without fail.
--[X] Things went wrong, get Sayaka and go to Oriko's. Insist if needed.
--[X] Get Kirika to enchant everything you might need to antimagic Riona, Gem and body.
--[X] Explain eveything. Avoid Social Bombs.
-[X] Plan (listen to opinions):
--[X] Simultaneous:
---[X] Blast Riona and her surrounding area with Anti Magic (Sayaka and Kirika).
---[X] Use Anti Magic restraints on Riona and her Gem.
---[X] Restrain Riona completely.
--[X] Mami and Homura know their roles better than you do.
--[X] Warn Sayaka to not try copying Riona (danger of copying
Soulguca).
-[X] Enact plan.
[X] If Riona's successfully detained, don't drop timestop.
-[X] Pull Sayaka aside. Make sure to not be overheard. Ask whether she decided to learn the secret or not.
--[X] Do your best to not pressure her.
-[X] If she says yes, carefully explain the Witchbomb, with emphasis on the fact you're
fixing it, and that Sayaka can help.
--[X] Ask Sayaka to copy Riona (while Riona's still Anti-Magicked).
---[X] If that works, ask her to try and take control of the shades for future reviving.
---[X] If it doesn't work, it doesn't work.