I spent some time trying different umbra boss comps in the arena. For me, this one is still the best BY FAR, the chink didn't lie.
Caramel is worthless in the boss fights, on any slot, with any build. Don't listen to Caramel propaganda, anyone telling you to use her is trying to grief you. Her being able to pop marks doesn't make up for her almost negative output.
Coronis isn't bad as main, but she has the same problem as Misty. She's just much, MUCH stronger on the support slot. I tried every variation, and Cosette vanguard/Coronis support performs better, even without the ability to pop marks on any unit.