Where to report russian resident

Title basically, the person performing ddos on our website was actually dumb enough to contact us via discord and ask for money. It appeared that we worked with him in the past about 5 months ago and we had some mutual friends of which i found out his phone number too. Now obviously the phone number we got from another person and might not be right but discord should be able to have more info on him.

We reported this to discord abuse support and got a reply they investigating the matter. I know DDOS is against their rules and his account will be suspended in no time along with his server (around 4k members).

In addition to these actions, what is the best place or entity to report this if the person is living in Russia, what authority there deals with people performing these activities ?

You need to get legal advice or contact your local authorities. On Cloudflare you can only set your security level to IUA or block the connection based on specific details. Otherwise Cloudflare is not involved and cannot help.