The attackers were relying on the 'element of surprise'. [=they were relying on their attack being a surprise]
I tried living in the city, but I was/felt 'out of my element' there. [=living in the city did not suit me]
At school she was (really) 'in her element'. [=in a place where she was comortable and did well]
(..) I agree. But now time itself is of the essence. They cannot bring such a force, on foot, or by boat, to Wessex within a week! Even so, the element of surprise is the most valuable asset. We... Father, since you've given me command, then let me exercise it. Father, I want to fight with you. As do I. (..)2017 TvShow: Vikings Title: On the Eve Season: 4 Episode 19