XML : How to return a big XML response in C# Web API

I am creating a web api which need to create large XML responses according to various requests. I tried googling but all the answers are about XMLSerializer.
And I tried making XDocument object and returning it using toString() method. But it's not working properly too. It gives me output like this

  <string xmlns="http://schemas.microsoft.com/2003/10/Serialization/"><root> <someNode>someValue</someNode> </root></string>    

Isn't there an easy way to generate XML dynamically and return it.