be out of depth(meaning someone has a problem that he does not know how to solve (АБ) Berezitsky); beats me(Beats me how he does it. – Ума не приложу, как он это делает. • It beats me how they finished before us. • It beats me why she goes out with him.Soulbringer)
be at one's wits' end("At last the little money which I had saved began to run short, and I was at my wits' end as to what I should do." (Sir Arthur Conan Doyle)ART Vancouver)