a tough nut to crack
1. A difficult task to complete. Getting an A in this class will be a tough nut to crack. You want me to find more money in the budget? Ha, there's a tough nut to crack!
2. A challenging and/or unreasonable person to understand or deal with. Our super-strict principal is a tough nut to crack, so I hope Kate can convince her to let us host this event. Because our boss usually keeps to himself, I have no idea what his interests are—he's a tough nut to crack.