Magento – How to apply free shipping promo code to only one shipping method

Create a module with the following config.xml:

<?xml version="1.0"?>
<config>
    <modules>
        <Shipping_Onepage>
            <version>1.0.0</version>
        </Shipping_Onepage>
    </modules>
    <global>
        <blocks>
            <checkout>
                <rewrite>
                    <onepage_shipping_method_available>Shipping_Onepage_Block_Method</onepage_shipping_method_available>
                </rewrite>
            </checkout>
        </blocks>
    <global>
</config>

This is the rewritten class in Shipping/Onepage/Block/Method.php, which removes all other methods if free shipping is available:

   <?php
    class Shipping_Onepage_Block_Method extends Mage_Checkout_Block_Onepage_Shipping_Method_Available
{
    public function getShippingRates()
    {
        $rates = parent::getShippingRates();
        if (array_key_exists('freeshipping', $rates)) {
            $rates = array('freeshipping' => $rates['freeshipping']);
        }

        return $rates;
    }
}

Source: http://www.blog.magepsycho.com/hide-other-shipping-methods-when-free-shipping-is-enabled/

You can set rules as

In Condition

You can use assign rule for each sku

In Action

select Free Shipping For matching items only