How do I create a specific login page for a specific user?

On-topic answer:

You can just put <?php wp_login_form(); ?> into any of your theme templates to render a login form on the front end of your site.

Or make your own shortcode [loginform] by putting this into your theme’s functions.php:

function wpse_242473_login_form() {
    return wp_login_form( 'echo' => false );
}

add_shortcode( 'loginform', 'wpse_242473_login_form' );