There’s a way to scale media (images) at 50%?

You can use image_downsize filter and to catch when WordPress wants a downscaled image and not original, and actually a size what doesn’t exist.

add_filter( 'image_downsize', 'wpse_60890_retina_scale', 10, 3 );

function wpse_60890_retina_scale( $value, $id, $size ) {
    if ( $size == 'wpse_60890_retina_scaled' ) {
        if ( !is_array( $imagedata = wp_get_attachment_metadata( $id ) ) )
            return false;

        $img_url          = wp_get_attachment_url( $id );
        $img_url_basename = wp_basename( $img_url );

        if ( isset( $imagedata['sizes']['wpse_60890_retina_scaled'] ) ) {
            $intermediate = $imagedata['sizes']['wpse_60890_retina_scaled'];
            $url  = str_replace($img_url_basename, $intermediate['file'], $img_url);

            return array( $url, $data['width'], $data['height'] );
        }

        $file = get_attached_file( $id );
        $image = wp_load_image( $file );

        if ( !is_resource( $image ) )
            return new WP_Error( 'error_loading_image', $image, $file );

        $size = @getimagesize( $file );
        if ( !$size )
            return new WP_Error( 'invalid_image', __('Could not read image size'), $file );

        list( $orig_w, $orig_h, $orig_type ) = $size;

        $max_w = round( $orig_w / 2 );
        $max_h = round( $orig_h / 2 );

        $resized = image_make_intermediate_size( $file, $max_w, $max_h );

        $imagedata['sizes']['wpse_60890_retina_scaled'] = $resized;

        wp_update_attachment_metadata( $id, $imagedata );

        $url = str_replace($img_url_basename, $resized['file'], $img_url);

        return array( $url, $resized['width'], $resized['height'] );
    }

    return $value;
}

Now you have to determine where you need full size or scaled by 50%.

For example you have

   echo 'full size:' . wp_get_attachment_image( $attachment->ID, 'full' );
   echo 'scaled by 50%: ' . wp_get_attachment_image( $attachment->ID, 'wpse_60890_retina_scaled' );