Optimize my Wordpress site running on Litespeed Web Server

My wordpress site always goes down (gives 500 Internal Server Error) during high traffic when it's over 2,000 users online. It needs to handle up to 10,000 at least. My dedicated server should handle that easily. Specs for the dedi server are, Intel Xeon E3-1240V2 24GB RAM Intel 520 Series SSD cPanel/WHM Linux CentOS But it doesn't handle the high load. I've tried hiring an expert to use Nginx and Varnish. It didn't do anything. Currently using Litespeed web server and it still went down when I had over 2,000 users online. Can someone configure so it can handle that much high traffic?. I've used WP-Super Cache and Image Optimizer plugin as well and doesn't help at all.