We didn't do the elimination diet exactly but we did eliminate gluten from my difficult child's diet as a result of a family history of celiac disease and gluten intolerance. We saw an immediate improvement in her behaviour. When we started seeing problems again, we realized we also needed to eliminate casein. We had testing done on her for gluten and casein so we knew to start with those. Now that she is gluten and casein free, she is truly a easy child. However, the old difficult child does make an appearance when she has even traces of either gluten or casein.
Before we started this diet, she was a true difficult child with a diagnosis of ODD and depression. She was taking 20 mg of Lexapro and looking at adding Seroquel or something else. One of our worst momements, pre-medications, was when I was trying to get her to go to her room for some infraction and she bit me. She was probably 9 at the time. Now she is off from all medications and doing better than ever.
I am about to do a food allergy test on easy child to see if there is anything else she is allergic to. This is not generally accepted by most doctors but they don't generally accept that food will cause behaviour problems either. I will use it as a starting point for eliminating other foods. There are several blood tests that use different methods that you can use. I am ordering it online but there are doctors and naturopaths that will do it also. You can look online for food allergy testing to decide which one to use and then ask them for a doctor in your area.
If you want to do it on your own, you could start with the top 8 allergens. You could eliminate them one at a time or all at once for a couple of weeks, observe behaviour, then add one thing back in at a time, and observe behaviour. I would check gluten and not just wheat. The top 8 allergens are wheat, milk, soy, eggs, peanuts, tree nuts, fish, and shellfish. Other good ones to check are artifical colors and high fructose corn syrup.