Couldn't agrrr with SirK more...
Experience is by far the best teacher, which is exactly why it is best to get a good amount of it BEFORE attempting some of the more delicate plants. A good understanding of growing plants in general, goes a long way. Okay, enough said about the folly of decisions that were already made...
....as far as the plant is concerned, the advice of trimming off (with a SHARP small scissors...and not a knife or by pulling!) the dead material, getting it out of a tray of water (if one exists), cutting back on the water and increasing ventilation is good solid advice.
I would go further myself, but not knowing the actual plant growing experience you have, I cannot recommend further action. I have seen cephs come back from worse, however as this damage has occurred in less than a month, and to a large plant that was not recently repotted, I also suspect that there is more problems with the environment it is experiencing than we are aware of.
Exactly what kind of light "bulbs" do you have this under. They could be part of the problem also as was mentioned. (I am not sure what you have exactly, and am really not sure why you chose them, if indeed they are not a normal/good choice.)
But then I also don't understand why you are throwing bugs at a plant that obviously can't make use of them, and this again falls back on learning about growing these plants before actually subjecting one to the wrong environment and then randomly doing the wrong thing with the hopes that something is right and will help it.
Using Trichoderma might have been a better choice in my opinion, and I have had good luck with it. It might help salvage the situation here, although it is hard to say.
I think more patience and less water is a good start.
I know about the urge to water it when you see it dieing, but it is the wrong thing to do.
And again, I would offer more advice, but to do certain things required a bit more experience to do it well, and otherwise one would likely only make things even worse.
Good luck, you are gonna need it.
(I have seen Cephs in worse shape come back to life, so it isn't hopeless. But indeed you need to get the environment into what the plant needs!
Sad to see such a nice plant decline so fast.
But it means something.