List irregular S1914 here. I will start:
(1) Railgun Trenchline:
-Find a chokepoint, or, failing that, a border you can entirely cover with railgun fire.
-Wait for the enemy to attack, disable the border province railroads
-Position the railguns slightly farther back and put artillery in front along with infantry (close enough to shell your own captured provinces, but not close enough to be shelled by enemy railguns in their provinces)
(2) Border Artillery
-Place an artillery on a "road" that puts it within range of the road the enemies are crossing, but also able to attack them once they reach your city.
-Bombard as much as required.
(3) Political Coup
-An ally has left the game/you want to betray an oblivious, weaker ally quickly?
-Put about 10 troops in each province (depending on number of forts/troops in province)
-Once all provinces are covered, declare war
-Done correctly, you should basically gain the entire country more/less intact without damage to railroads/factories/etc.