8 AI on the smallest map on easy. I get lucky by having only 2 chokepoints between me and the mighty yellow empire. I close the left-most entrance as much as I can (to the point where I don't think AI would use it), and concentrate my meager defenses to the north.
http://img827.imageshack.us/img827/5005/elemental20100829001739.jpg
http://img28.imageshack.us/img28/9224/elemental20100829001729.jpg
http://img205.imageshack.us/img205/2443/elemental20100829001716.jpg
http://img844.imageshack.us/img844/3190/elemental20100829001648.jpg
Because Arma2 OA is an awesome game, and I somehow survived a several second long machine gun burst.
http://img196.imageshack.us/img196/7749/arma2oa2010082811313524.jpg