There are many articles and books written on how to deal with difficult customers. I know you may think this is naive but a better question for me would be
“Why do you have to deal with difficult customers?”
90% of the time you don’t have to. There are exceptions but barring those (very few) exceptions, a sure-fire strategy (pun intended) to deal with difficult customers would be to fire them.
The second best strategy (tongue in cheek) would be to refer them to your competition 
Have a great week ahead!
