Okay, if anyone attacks Oni, they shouldn't do it physically.
Canaan- Gun is non-physical, but we probably want to keep her on Maria protection duty. I think having her hide away with Maria is probably the best option, because then Canaan is freed up in the following turns and it's a contested trickery, which she's quite good at.
Clove- She only has physical attacks, but can also coordinate with Belle. May as well do the warehouse search, helps keep her out of danger while her HP is low.
Does her combo move actually get full advantage if we pick it though? Or does it cancel out with the oni's trait?
Belle, well, I wanted her to loot the warehouse anyway. Better chances of getting something good coordinating with Clove and being directly helpful.
Metal has blasting, which leaves his other AP protecting Belle, I guess. Plus healing Clove for his free action.
Piastol, thankfully, has two magic attacks in Welves and Eterni. Plus potentially healing Clove.
THAT SAID, if Metal is also probably healing her and Piastol's heal will need to recharge for the trade-off of automatically succeeding, so I might consider saving it in case they mess up someone else. Instead, especially with so many attacks probably going for Oni, might be worth trying to debuff him with Noxi.
Vector has some GOOD options here, between music attacks (AoE!), bubblegum and fire breath.
Espio, may as well just use his ninja arts.