hard nut to crack

Short definition from Wiktionary

Noun

1. 1
   A problem that is challenging to solve.
2. 2
   A situation, person, group, etc. which is difficult to overcome or deal with.
3. 3
   A place, opportunity, etc. to which it is difficult to gain entry.
4. 4
   An amount that is difficult to finance.
5. 5
   Used other than figuratively or idiomatically: see hard, nut, crack.