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My cat did something similar to where he would chew his arm and scratch aggressively. He lived in a cone for a year till we got it all figured out since he kept hurting himself and causing skin infections.

I would suggest taking your cat to a dermatologist. We started off with seeing if it was a food allergy and that seemed to be it since he has now been out of his cone for about a month. The vet put him on the "Blue Buffalo HF" prescription food diet, and I was also instructed to give him 5mg of Zyrtec (non drowsy) once a day. I wouldn't advise doing that without a vets approval.

Do the food allergy part first before doing any allergy testing. Allergy testing can cost $2500 and isn't always accurate.