You'll need to update your plugin code to do this the "proper way" I believe.
Since you need it to load last I would take the common path of utilizing the wp_enqueue_scripts hook/function to set a low priority for it being processed. This way you can guarantee that the HTML remains valid and that you are loading your styles and scripts after all the default ones within WordPress plugin's code:
function my_plugin_unique_style() {
$base = get_stylesheet_directory_uri();
wp_enqueue_style( 'style-my-plugin-style', $base.'/styles.css' );
}
add_action('wp_enqueue_scripts', 'my_plugin_unique_style', 11 );
Of course you will have to modify this to use your plugin's css file name but this is the basic way to do this and have valid markup. It's worth mentioning that if this still loads before another CSS file in the HEAD of the page you should bump up the number from 11 to some other higher number.
You can read more about wp_enqueue_scritps here.
add_action('wp_enqueue_scripts','testfunction',10);change any value from 1 to any value lower the number higher the priority