How to use Slerp / Lerp / RotateTowards to rotate an object (from-to) two rotations but around their Y axis only?

I've created a script for character movement from scratch .. and it has just worked as perfect as i wanted, but I had a real issue..

in my script I've relied for movement on Slerp , Lerp and RotateTowards functions for Quaternions .. I mean I've tried the three of them and i got the same result of rotating and Looking towards camera just as I wanted .. The Problem is : my character is rotating around all the axis of my Orbit Camera [the target rotation] .. I just need it to be rotated only around a Y Axis of Orbit Camera so that when I rotate my camera up and down it doesn't affect my character to rotate down with it as well .. Take a look at my script :

 var Speed         : float = 6;
 var Gravity           : float = 20;
 var rotationSpeed     : float = 5;
 var Angle           : float;
 var MainCamera         : Transform;
 
 function Update() {
 
 controller = GetComponent(CharacterController);
 
 if (controller.isGrounded)
 {    
            moveDirection = Vector3(Input.GetAxis("Horizontal"),0,Input.GetAxis("Vertical"));
         moveDirection = MainCamera.TransformDirection(moveDirection);
         moveDirection *= Speed;
                  
         moveDirection.y = -10;
         
         if (controller.velocity.sqrMagnitude > 0.1)
            transform.rotation = Quaternion.Slerp(transform.rotation,MainCamera.rotation*Quaternion.Euler(0,Angle,0),Time.deltaTime*rotationSpeed);
         //
         if (Input.GetAxis("Vertical") >= 0.01)
         {
         Angle = 0;
         
         if (Input.GetAxis("Horizontal") >= 0.01)
         Angle = 45;
         
         if (-Input.GetAxis("Horizontal") >= 0.01)
         Angle = -45;
         }
            //    
         if (Input.GetAxis("Horizontal") >= 0.01)
         {
         Angle = 90;
         
         if (Input.GetAxis("Vertical") >= 0.01)
         Angle = 45;
         
         if (-Input.GetAxis("Vertical") >= 0.01)
         Angle = 135;
         }
         //
         if (-Input.GetAxis("Vertical") >= 0.01)
         {
         Angle = 180;
         
         if (Input.GetAxis("Horizontal") >= 0.01)
         Angle = 135;
         
         if (-Input.GetAxis("Horizontal") >= 0.01)
         Angle = -135;
         }
         //                                                                                                 
         if (-Input.GetAxis("Horizontal") >= 0.01)    
         {        
         Angle = -90;        
         
         if (Input.GetAxis("Vertical") >= 0.01)
         Angle = -45;
         
         if (-Input.GetAxis("Vertical") >= 0.01)
         Angle = -135;        
         }            
 }      
 
 // Move the controller
 controller.Move(moveDirection * Time.deltaTime);
 }
 
 function ApplyGravity () {
     moveDirection.y -= Gravity * Time.deltaTime;
 }

Please , help me in my problem within these three functions Slerp or Lerp or RotateTowards, Thank you.

here's a sample of the game on the web so that you could see the problem by your selves : http://www.4shared.com/rar/jdycCqeS/The_Sample_Game.html?