He's a hard nut to crack.

他是一個難對付的家伙。

“hard nut to crack” 先理解一下表面意思：

hard 硬的，在這里用來強調 nut 很硬；
nut 堅果，表示堅果有硬殼很堅硬，不易砸開
crack 裂縫，聲變，破裂，變聲

nut n.1 堅果，堅果仁 2難對付的人，難事，難題

crack vt 發出爆裂聲，使爆裂，使破裂

字面意思是他是一個很難砸破的硬堅果，但人怎么能是一個堅果呢，明顯是一個引申義，所以翻譯成他是一個很難對付的人。

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.
眾所周知,我們的老板是個難纏的人. 我們每次想請假,他都說不行,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: 你怎么看起來那么苦惱?
B: 你不知道嗎 我們明天有一場數學考試
A: 那又怎樣?放松點,只是小菜一碟.
B: 對你來說是小菜一碟,對我來說，可是一場硬仗.