Hi Faizan,
I don't think the answer to this question can be found within Liferay. Liferay of course provides multiple layers of security, but they are all based around the application itself where as what you are experiencing feels to me more like a perimeter security issue. Most of the implementations I have seen use other products to protect the applications by analyzing the traffic and detecting DDoS or DoS attacks and then blocking the sourcef rom subsequent requests.
With that said, are there specific URLs that you see as being targeted? Can you share a little more information with us about your implementation, and what settings (portal-ext and hook) you are using/deployed? Perhaps there is something that you have enabled that is best disabled unless you need it.
