Yeah that's bacterial, the kind that rots the flesh and you are unlikely to save the ones severely infected. Columnaris can do this but so can pseudomonas and aeromonas which are associated often with anaerobic pockets. The latter two are more likely to cause internal bleeding and bloody ulcers on the body, columnaris can eat away at the flesh and fins and often has a white slimy film.
Salt might help, so can tannins. This is not likely protozoan in nature, they don't eat flesh like this. I'm hesitant on salt with cories, it's doable, but imo, tannins are better in this case. No matter the treatment, it's not likely to be stress free on them. I'd try tannins first, very heavy tannins as it is the least stressful option of all your options. Give it a couple days, if no improvement or it gets worse then go for salt, and if salt isn't good, you'll need kanamycin (kanaplex brand name). It's more effective for columnaris infections. You will never know 100% which bacteria it is without a microbiologist looking at it under microscope, but you can also take a more educated guess depending on how the infection behaves.
I've seen this before with hoplisoma leucomelas, ate the entire face away and the fish wasn't able to be saved as it was too severe. But there's hope for the ones not affected as heavy, but I'd be guarded on the one with the whole scalp eaten away. Interestingly, the leucomelas is in the same family as sterbai, hoplisoma, while bronze are osteogaster aenea, a different family. My leucomelas was in same tank at the time as osteogaster, and the osteogaster were also unaffected while the hoplisoma was. So it's very plausible that hoplisoma are more prone to this particular bacteria, but that's just a hunch, not anything I'd expect any studies done on.
