The basis of Shogun 2 is to ensure you have as many allies as possible. Towards the end of the game, once you grow in power, everybody will turn against you anyways. Until then, open diplomacy, get as many trade agreements going as possible, build only the forces required to defend yourself, upgrade your ports and let the money roll in.
If you're quick off the mark, also get the sea trade going to your favour, by sending ships to the off-map locations, to get better resources and more money.
Once your economy is stable, use assassins to to with those around you. If you manage to cause a revolt in a neighboring city, build up a force and take it quickly. Always occupy, instead of loot. That way you can get the city working stably again, gain more income, and move the army on as soon as possible to take the next city.
Playing aggressively on that game can be risky.
Finally, never leave your home undefended! I'm not saying to leave a whole army, but leave some scouting units behind, just in case a distant enemy decides to land on your shores, which, believe me, they will do often.