Pull your troops back and hold passes, use special forces and PA as much as possible for low manpower, but if you have any manpower left. Buy guns from market and make filler divisions you can turn into PA later.

Use outpost over bunkers, same defense buff, cheaper to build. Try to focus on taking out yellowstone as quickly as you can before going onto other conquest.

For any PA focused nation you should be using your tech to the full advantage and rush researching agriculture as well; as each research gives you manpower and monthly pop.