In computing, a denial-of-service attack is a cyber-attack in which the perpetrator seeks to make a machine or network resource unavailable to its intended users by temporarily or indefinitely disrupting services of a host connected to the Internet.
You can spin up thousands cloud servers for 2 hours and attach a server from several sources at specific time. Spot instance from AWS would be cheap. Since attack will come from different ips the target server will not be able to stop it easily.
Use different cloud providers, use different regions but start running the army of computer at the same time.