First, the writer should study the original Ben 10 series more closely, especially the female characters. Understand their personalities, their backstories, and how they interact with other characters. This will help in creating more accurate and engaging portrayals.
Get feedback from other fans. Join Ben 10 fan communities and ask for opinions on your work. They can point out what's wrong with your portrayal of the female characters and other aspects of the fanfiction. Listen to their suggestions and be open to making changes. This way, you can gradually turn the 'bad' fanfiction into something better.