I achieved this feat much easier that I'd have expected because of the following reasons:
1) The Strategic location of the Shimazu shogunate at the western edge of the country keeps my borders safe from other powerful clans such as the Hojo, Takeda, Tokugawa etc.
2) The the Western side of the country is also closest to the major trade routes, namely with the Indonesians, the Chinese, Koreans which became a vital source of income when secured.
3) The fact that the Shimazu specialized in Katana Samurai gave them a distinct edge in melee combat units as very few foot soldiers could stand up to an onslaught of veteran swordsmen, especially, most vulnerable of which are the Yari Ashigaru. Also the Shimazu have generals with higher than normal honor which makes generals less likely to sell-out.
4) A strong Navy is crucial to success. Capturing and protecting trade routes, patrolling the seas, disrupting enemy trade routes, blocking ports, and most especially delivering armies behind enemy lines.
5) Focus on economic growth, less on military at the onset most especially agriculture.
All in all, Shogun2: Total War was a source of hours of brain activity for planning and the satisfaction of crushing a rival clan made every minute worth it.
I'm still trying to figure out what game to play next. Maybe Dead Space 2? I've got a long list and not enough to cross all the games off them...
