Testimonial Rotator Archive Page Order By. Add this function to your functions.php file in your theme.

testimonial-archive-order.php

add_action( 'pre_get_posts', 'hg_change_testimonial_sort_order'); 
function hg_change_testimonial_sort_order($query)
{
    if( is_main_query() AND is_post_type_archive('testimonial') )
    {
	   $query->set( 'order', 'DESC' );
       $query->set( 'orderby', 'date' );   
    }  
}