sucker punch
be first when they don't expect it 
Problem with that is you're giving the opponent grounds to say you threw the first punch which might justify the ass kicking (mutual combat). It's cool in the movies when the good guy says, "My dad always told me, if you can't avoid an ass whooping, at least get in the first punch," as he clocks the leader. I don't think telegraphing your punch verbally is smart, though. Hollywood.

I prefer to be ready to block a punch without being the first aggressor. I keep one hand near the waist and the other below or in front of my chin as nonchalantly as i can. Had a guy try to kick me in the nuts once. Since I was already standing at an angle to him to keep my balance, I was able to twist my hips and protect my groin. At the same time, I reached down with the left hand (the hand I positioned near my waist) and grabbed his foot. I let him hop around on one leg for a few seconds before putting him on his ass -- just to fully enjoy his surprised expression.
I turned and casually walked away. I think he was too shocked (and maybe in too much pain?) to worry about chasing me. I do know he never tried anything with me again.
Nowadays, that would be on the Internet forever. He's lucky this was the pre-smartphone era.
BTW, that guy is a pastor now.
