Awkward/tricky/tough etc customer noun
Used to refer to a person who is difficult to deal with
This morning Nick helped me deal with a tough customer.
You should use the tough treatment for these tricky customers.
Someone who has a strong personality
It is difficult for someone (not) to do something.
To be difficult, troublesome, or unpleasant for someone; to be to someone's disadvantage.
Used to describe the action of treating someone very strictly.
If you say that someone or something is too hot to handle, you mean that they are too difficult or dangerous to cope with.
A person who is completely unknown or has no connection with you
The guy I talked to at the prom last night is a perfect stranger.