So.
We need to be wary of traps and unexpected enemies.
Our assassins go first, in stealth mode, scouting ahead. They both have good perception, so they can handle it.
Then, behind them (probably at a good distance, but close enough to see the scouts), we follow. Celia should definitely be at the front, and Castor closer to the back. I'm not sure how to position the others... In general, those who are slower and melee fighters should go forward. That is, Dillon. I'll be a little behind, and our mages will be at the very back. Should everyone be in pairs? I think that's better,
so we don't spread out the formation too much, and we can protect each other from possible threats from the side/back.
I think it would be best for you to have the sorceress Nebka at the very (or almost very) back, so she can hit everyone with her beam. Maybe put her second to last, so Ayano closes the procession. You have better evasion, in case of an attack from behind. And you won't get much benefit from healing. If someone manages to break through the spiritual barrier, you'll only have enough health for one average attack.