like getting blood from a stone
Impossible or extremely difficult to accomplish. Good luck getting a group of toddlers to sit still—it's like getting blood from a stone. A: "Why don't you ask your dad for the money?" B: "Because getting my dad to lend money is like getting blood from a stone."
like getting ˌblood out of/from a ˈstone
extremely difficult to obtain: Getting an apology from him was like getting blood out of a stone.