Brute Force Attacks against WordPress have always been very common.
Here is how to temporarily stop a Brute Force Attack exploiting XML-RPC (xmlrpc.php) in WordPress by adding the following code to your .htaccess file:
# Redirect WordPress xmlrpc.php requests
RewriteRule ^xmlrpc\.php$ "http\:\/\/0\.0\.0\.0\/" [R=301,L]
# Block WordPress xmlrpc.php requests - Cause high CPU load
deny from all
#allow from 188.8.131.52 # Uncomment and add your IP if required