using System;

using System.Linq;

using System.Xml;                   

using System.Xml.Linq;

using System.Collections; 

using System.Collections.Generic;

using System.Data;

using System.Data.DataSetExtensions;

using System.Data.Linq;

using System.Text;

using System.Text.RegularExpressions;

using System.Xml.Serialization; 

using System.IO;

using System.Globalization;

​

public class Program

{

    public static void Main(string[] args)

    {

        string input = @"<root>

<level1 id=""NotNeeded"">

</level1>

<level1 id=""Needed"">

    <level2 id=""Needed"">

        <level3 id=""Needed"">I need this one</level3>

        <level3 id=""NotNeeded"">I don't need this one</level3>

    </level2>

    <level2 id=""NotNeeded"">

        <level3 id=""NotNeeded"">I don't need this one</level3>

    </level2>

</level1>

</root>";

        XDocument doc = XDocument.Parse(input);

         doc.Descendants()

            .Where(x=>x.Attribute("id") != null && x.Attribute("id").Value == "NotNeeded")

            .Remove();

        doc.ToString().Dump();

    }

}

​