“a hundred days of sermonizing, one fart” → a metaphor for how something that one has put great effort into can come to nothing after even a slight mistake, much as a hundred days of serious spiritual study can be lost to the winds

(historical) Aisin Gioro, the Manchu family which established and ruled China's Qing dynasty.

(literally) know the other side and yourself, a hundred battles will not be perilous