a hard egg to crack
A person, thing, situation, or problem that is particularly difficult to understand, solve, or deal with. I've been dating Jenny for over a year, and I still think she's a hard egg to crack! Figuring out the best way to modernize our product without alienating existing customers is definitely a hard egg to crack. The spy we captured is a hard egg to crack—he hasn't said a thing since we began the interrogation.