I'm pretty good at beating my computer at chess, even though I'm an awful player. I challenge it, and it runs out of time - apparently it can't tell that it's in a competition, or can't press the button on the clock.

This might sound like a facetious answer, but I'm serious. One way to defeat something that is stronger than you in a limited domain is to strive to shift the domain to one where you are strong. Operating with objects designed for humans (like physical chess boards and chess clocks) is a domain that current computers are very weak at.

There are other techniques too. Consider disease-fighting. The microbes that we fight are vastly more experienced (in number of generations evolved), and the number of different strategies that they try is vastly huge. How is it that we manage to (sometimes) defeat specific diseases? We strive to hamper the enemy's communication and learning capabilities with quarantine techniques, and steal or copy the nanotechnology (antibiotics) necessary to defeat it. These strategies might well be our best techniques against unFriendly manmade nanotechnological infections, if such broke out tomorrow.

Bruce Schneier beats people over the head with the notion DON'T DEFEND AGAINST MOVIE PLOTS! The "AI takes over the world" plot is influencing a lot of people's thinking. Unfriendly AGI, despite its potential power, may well have huge blind spots; mind design space is big!