Same recipe every game.
Get as many metsuke guys (the ones that can assasinate and control the populace, bribe, etc). One of their level 6 abilities gives +1 happiness clan wide. So if you build them up from the beginning you will get +5 happiness global which helps tremendously, it means at least 1-2 ultimate tier economic buildings per province.
Blitzkrieg up until 14-18 provinces, or earlier if you come across one that has a gunsmith/blacksmith.
Upgrade blacksmith all the way, get training camp> firing range, upgrade all the way infantry and cannon buildings.
Min/Max economy on all other provinces, you might want a police station and an inn for happiness control. When in happiness surplus, upgrade inn to market and get cottage industry, farms past commercial farming if fertile, trading ports, etc.
By around turn 120. I am rolling on 20k per turn. With 4-5 full stacks, all from the blacksmith province. It becomes a roflstomp afterwards I usually lose interest. The cannons get around 90 accuracy off the factory. And if you add a lvl 5 foreign veteran with the accuracy and reload skills as well as the general... well; you will hardly lose more than 30 men per battle. Right now its about run 190 on VH, and I'm rolling on 55k a turn lol with 3 maxed out navies, 6 all out stacks... lol