8 year old cat very allergic to all flea poisons.

My poor old cat was introduced to fleas for the first time. Vet applied Frontline in office visit. She chewed or scratched all the hair off the spot where it was placed. She was scratching all over also and her hair stood up all over her body. Had a cortisone shot. Helped for awhile. Then the fleas were back. Tried a Seresto flea collar. She was also scratching at the collar all the time. Fleas finally eliminated from the home. Her hair is till standing up and she is still biting the middle of her back months later. I bought 2 of the collars for both cats at $60 apiece. This got the fleas finally. She is a Russian Blue breed. A cat of the same breed lived 23 years with us and also had this allergy. What can I do to make her more comfortable without cortisone injections?