First of all you should still not rule out a food allergy - "natural" means nothing if your dog is allergic to one or more of the ingredients. The most common food allergens are beef, corn, chicken, wheat soy and dairy, and sweet potato is becoming more common as well. So is pork. I know one dog that is allergic to oatmeal...
Note that chicken is the first ingredient in this food, and it is followed closely by duck - if you squint and it sort of looks like a chicken, and she's allergic to chicken, then she's probably also allergic to duck. Try switching over to one of the Wellness Simple Solution or Core foods that is only fish or lamb, and see if it makes a difference. And remember to eliminate ALL other things like chew hooves, bones, pig ears, bully sticks - all body parts, as well as cookies and otehr treats that could contain allergenic ingredients. The name of the game is to find one thing she CAN have, and then see what else she can tolerate. But man allergy dogs just get a few of their limited ingredient kibble as a treat.
There is a creme rinse called Resicort that has a small amount of cortisone in it, and a bottle of that should last a Chihuahua a very long time. But bottom line, you really should work to make sure you have eliminated all allergens, rather than treating her for a reaction after the fact.
If you create a safe zone for her by making sure there are no fleas in the house, the yard is treated, and she has Frontline applied on time, she should be safe from that allergen. But more often than not, the underlying problem is the dog's own immune system, and they tend to react to many things, not just one. Until you identify all of the things to which she may be reacting, and remove them from her world, she'll still be shoveling against the tide.
This isn't easy, but so long as you control her environment, you can limit it and see if she responds with less reaction. THis is what veterinary dermatologists do, and it is even more reliable than skin tests. Write every thing down, and read, read, read the ingredients. Once you find out what she needs and what she should stay away from, your dog won't need any steroids at all.
Lowell Ackerman, Tufts Cummings Veterinary School Dept of Dermatology, and experience working with many allergy dogs myself.