Yeah, that's how I played my France when I got Himeji Castle. Earn promotions in your own territory, then when you get Foreign Legions (or Mech Infantry with the promotion carried over) you switch to the offensive. Pretty tough for the AI to stop. Ironically, you do better if the AI opponent is moderately strong (lots of battles = lots of promotions, and they have more cities to take) than weak.
I did learn from those trials that unless your AI neighbour is a juggernaut, Himeji Castle isn't necessary to keep safe (Oligarchy alone is fine). It might be more helpful post-patch.