Attack them when they are unready, take them by surprise ： 这个成语有多少方法可以说？
Usually you can use any of them, but &quot;攻其无备，出其不意&quot; is the best one.
The reason is this word is from 《孙子兵法》, an ancient military book by a famous military strategist Sun Wu(孙武), and it is &quot;攻其无备，出其不意&quot; originally in that book.
Also, in 《现代汉语词典》（第7版）, there is &quot;攻其无备，出其不意&quot; under &quot;出其不意&quot; as an example.