tough nut to crack

Wiktionary

1. n. Used other than as an idiom: see tough,‎ nut,‎ crack.
2. n. idiomatic A problem that is challenging to solve.
3. n. idiomatic, by extension A situation, person, group, etc. which is difficult to deal with.
4. n. idiomatic A place, opportunity, etc. to which it is difficult to gain entry.
5. n. An amount that is difficult to finance.