boom.php

<?php

function my_pre_user_query( $query ) {
	$where = get_posts_by_author_sql( 'post' ) . " AND MONTH(post_date) = " . date( 'm', strtotime( '-1 month' ) );
	$query->query_from = str_replace( get_posts_by_author_sql( 'post' ), $where, $query->query_from );
	$query->query_where .= " AND post_count > 0 ";
	$query->query_limit .= " LIMIT 5 ";
}

function my_magic_loop() {
	
	// Get last month's top authors.
	add_action( 'pre_user_query', 'my_pre_user_query' );
	$top_authors = new WP_User_Query( array(
		'orderby' => 'post_count',
		'fields' => 'ID',
	) );
	remove_action( 'pre_user_query', 'my_pre_user_query' );

	// Get last month's top authors' posts.
	$top_posts = new WP_Query( array(
		'post_type' => 'post',
		'author' => implode( ',', $top_authors->get_results() ),
		'monthnum' => date( 'm', strtotime( '-1 month' ) ),
	) );

	// Boom!
	if ( $top_posts->have_posts() ) {
		while ( $top_posts->have_posts() ) {
			$top_posts->the_post();
			// ...
		}
	}
}