score:9

Accepted answer 
String input = "3030313535333635";
String result = "";
for(int i = 1; i < 16; i +=2 )
{
    result += input[i];
}

score:12

Try this to get the every other character from the string:-

 var s = string.Join<char>("", str.Where((ch, index) => (index % 2) != 0));

score:6

You can use this well-known class System.Runtime.Remoting.Metadata.W3cXsd2001.SoapHexBinary :)

string str = "3030313535333635";
var hex = System.Runtime.Remoting.Metadata.W3cXsd2001.SoapHexBinary.Parse(str);
var newstr = Encoding.ASCII.GetString(hex.Value);

score:1

Code in Java Language

String input= "3030313535333635"
String output="";

for(int i=1;i<input.length();i=i+2)
{
    output+=input.charAt(i).toString();
}
System.out.println(output);

score:3

Using a StringBuilder to create a string will save resources

string input = "3030313535333635";
var sb = new StringBuilder(8); // Specify capacity = 8
for (int i = 1; i < 16; i += 2) {
    sb.Append(input[i]);
}
string result = sb.ToString();

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