He's a hard nut to crack.

他是一个难对付的家伙。

“hard nut to crack” 先理解一下表面意思：

hard 硬的，在这里用来强调 nut 很硬；
nut 坚果，表示坚果有硬壳很坚硬，不易砸开
crack 裂缝，声变，破裂，变声

nut n.1 坚果，坚果仁 2难对付的人，难事，难题

crack vt 发出爆裂声，使爆裂，使破裂

字面意思是他是一个很难砸破的硬坚果，但人怎么能是一个坚果呢，明显是一个引申义，所以翻译成他是一个很难对付的人。

This sentence is used to describe a person. If someone is powerful, you cannot easily beat him. This sentence is said by you. Although you are not easy to please still, you will not lose your heart.

例句：
As we all know, Our boss is a hard nut to crack, every time we want to ask for a leave, he simply says no, whatever reason you might have.
众所周知,我们的老板是个难缠的人. 我们每次想请假,他都说不行,不管你有什么理由.

A: Why do you look so upset?
B: Don’t you know that we are going to
have a math exam tomorrow?
A: So what? Relax. It is a piece of cake.
B: It might be a piece of cake for you, but for me, it's a hard nut to crack.

A: 你怎么看起来那么苦恼?
B: 你不知道吗 我们明天有一场数学考试
A: 那又怎样?放松点,只是小菜一碟.
B: 对你来说是小菜一碟,对我来说，可是一场硬仗.