I rather play surrounding the area I want, then go inside, the good thing is that I can make several attacks on the same box. This strategy includes a defensive part, put as less troops as you can in your border, and several units nearby this border key points. If they attack, they will win for sure, but then you have the upper hand for retaliation, destroying his whole task force...damn, this topic is so out of section, either arts and entertainment Risk, or should be in Game tips.