hard nut to crack

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Used other than as an idiom: see hard,‎ nut,‎ crack.
• noun idiomatic A problem that is challenging to solve.
• noun idiomatic, by extension A situation, person, group, etc. which is difficult to overcome or deal with.
• noun idiomatic A place, opportunity, etc. to which it is difficult to gain entry.
• noun idiomatic An amount that is difficult to finance.