Defend with the utmost agression

We've already mentioned the element of surprise, which usually is initially with the attacker. He expects a certain behavioral response on your part now. The best thing you can do in this situation is the exact opposite of what he expects, which coincides with the reaction that he prefers the least:

You're going to attack your attacker with everything you've got.

You will go "mental". This doesn't mean that you start to swing your fists around like a monkey on drugs, but instead you will ferociously and viciously attack the weak points of your attacker.

Attackers HATE to be attacked! They are not used to having to defend themselves.

Knowing the weak points of your attacker is already half the fight won. Most wanna-be thugs or attackers are not even aware of their own weak points (except maybe their groin) so they wont make much of an effort to protect these areas when launching an attack.

This is your advantage!

You won't fight in the way your attacker expects you to fight, namely throwing around fists like randy cowboys. No, you will unceremoniously use simple and effective techniques to instantly stop the attack before you get hurt.

Society conditions us to use a measured response, if any. We don't want to hurt anyone, not even our attacker, so we may just want to block a few attacks, move away and hope he will change his mind and cease his attack.

This is VERY unlikely.

In simple words: you must attack until there is absolutely no more immediate physical danger from your attacker. Let's repeat this: Keep attacking until you're absolutely sure there is no more danger from your attacker.

This means, you will not just throw a few punches. No, you will viciously attack the weak points of your attacker until you're sure you've created enough of an opening to allow for a successful escape.