A tough (or hard) nut to crack is a problem that's hard to deal with, or a person who is difficult to understand.
My new boss is a tough nut to crack. I never know if he's pleased with my work.
How can we increase sales during a recession? That's a tough nut to crack!
To use a sledgehammer to crack a nut means to use more force than is necessary to achieve a goal.
The police sent five officers to arrest one teenage thief? That's really using a sledgehammer to crack a nut!
In the ancient Indian language of Sanskrit, the coconut palm is known as kalpa vriksha - 'tree which gives all that is necessary for living' - because nearly all its parts can be used: the shell, flesh, water, milk, sugar and oil.