「棘手问题」英文怎么说？

a tough nut to crack 

难对付的人、棘手的问题、难办的事

Acing the test will be a tough nut to crack. 

想考到好成绩是一个很棘手的问题。

My boss is so demanding. Man ! A real tough nut to crack. 

我的老板太苛刻了，哎！真的很难相处。

The intro 

（Listening Comprehension）

Hey ! It's Mr.Wong speaking, and today's topic is about nuts. I'm not talking about ordinary nuts. These nuts are hard, they're tough. And no ! Not those kinds of nuts you guys are thinking about. I'm talking about the nuts we use in idioms."A tough nut to crack" ~ which means a difficult task to complete, a challenging or unreasonable person to understand or  deal with. 

没看懂？请自己查字典！

（这个才是真正的学习过程）