Best army, Building, Christianity. (Total War: Shogun 2)

Already played more than 100 hours, was Shimazu on average katana and Arquebusiers, but on legendary I can not pass, the same tactic almost never works. I want to know:
1: Is the army (an army for example 1полк., and 19.....) which auto calculations and win on the battlefield all cut?
2: House is the house of IMBA? For example, Otomo everything seems OK, but CHRISTIANITY! Or a house in which there are some Legendary the troops?
3: Christianity, whether smysel such as for Shimazu to accept Christianity? Or with date?