Avoid the dogs. Use barrels whenever possible to take out large groups of soldiers\dogs. Often in these sort of missions also, there is usually some form of POW section for you to recruit reinforcements. Take the time to kill\blow up everything you can safely manage to and see what happens.
Wow. You have nothing but a single spy. I forgot this was the mission you were mentioning because it's early and I'm tired. I apologize, in this mission your best bet is patience. Many of the patrols have different paths if you wait long enough to see them, allowing you time to pass. The part about finding POW's still applies, only you cant blow anything up so only your presence would be required to free the prisoners. I can't remember the exact specifics of this mission as it has been too long but that should be the gist.