How to prevent the F5 from DDoS?

ASM DOS Protection Profiles and ASM Security Policies are not linked to one another. You can use a DOS Protection Profile without ASM Security Policy. The DOS Protection Profile you create must be applied to all related Virtual Servers if you want it to have a global effect across all of your services.

Some basic TPS-increase based rules which are available in the ASM module will suffice for mitigating smaller attacks. In case of a large event, you will need a cloud-based solution to clean up the incoming traffic. F5 has Silverline in their offering.