tall order

a tall order AND a large order (something very difficult that someone expects you to do) — чрезмерное требование; трудная задача, нелёгкое дело; трудновыполнимое задание, требование

Example 1: That's a pretty tall order. — Вы много от нас хотите. Это нелегко сделать.

Example 2: You want 2000 items delivered by Friday? That's a tall order. — ...Это будет нелегко сделать.

Example 3: Going by what Barash has to say about religion, Darwinian thinking isn’t likely to transform our understanding of it anytime soon. We do not even know why we are relatively hairless or why we walk on two legs, so finding the origin of religious belief is a tall order. (newyorker.com)