The modern diagnosis for children of ADHD, Autism, and ODD are useful tools with many effective strategies and treatments to help cure them, or at least manage them well. There is an older concept in psychology that is also helpful to keep in mind when dealing with a child who has some conduct and behavioral issues.
Often children do pick battles of wills and create unnecessary beliefs about competition with peers and even parents. Aspects of society even encourage this in ways. Still, the child is generally seeking love and understanding and certainly needs to learn cooperation as a tool to engage the world with. This is where the balancing act begins. Neither being too strict or lenient is ultimately very useful. Being curious who your child is and allowing them to tell you who they are in ways that are not challenged or dismissed is often vital. These connections can be created by time spent “being” with your children, instead trying too hard to make them “do” or “learn” something.
I know this is not always a popular take on how to correct children. Our society often stresses giving orders to others, and we are in the habit of doing that towards children. In turn, the child often “dismisses” us in some form, which hurts our ego and sense of authority. I would say there are ways to let the child know that you do have feelings and can feel vulnerable to their “willful” behavior. This gives them a chance to be compassionate which often is a great outlet for the humanity we all need.
Throw a protest - have you ever said to your child, "No, I'm not going to play with you. Even though I want to play fair with you, since you are being too much of a brat, it's no fun if you always change the rules so that you always win." Overall, I am expressing an attitude that I have discovered, through hard won effort in play therapy that has greatly reduced the aggression of several “willful” children in play therapy. Frankly, I found this method by accident because some children were way too “willful” for me! Lol. Each parent can adapt this general attitude toward communication methods that fit best. See your child as person who needs protection, but also courage in a society that is giving very, very mixed, and confusing messages. I hope this helps. It’s an idea for an approach to a new start.