Something very simple and obvious that keeps on proving to be very useful is, when clients ask for not-feasible or really complicated or compromising requests, show yourself receptive and willing to help them achieve what they want; trying to stay positive even though it seems to you there's nothing you can do about what they ask. Then, looking for alternatives and being clear about the options you can provide to match their requests as much as possible is very often enough for them to calm down and feel they're in good hands because they see you are putting the effort they expect from you, and they might actually like one of your alternatives better than their fist idea.

One thing I've found helpful is matching a difficult customer's energy. It helps mirror their behavior back to them and makes your attention seem as important as their problem.

When you come across a Challenging Client. A Skill you must make active is listening. Listening to hear them speak, express themselves. While listening, ensure you are taking note of the exact problem they are trying to communicate with you.

Allowing them time to speak while actively listening. Assess their needs while keeping positive communication. Looking to find a solution.

Listen to the client in first had to understand the problem. After a good communication either to inform him about rules and what can or cannot be done as a solution or simple information. The client normally considers to have a certain level of reason to discuss, to question or to say something is not according. We need to understand this reality, but also the reality of the situation will determine, till where can we go with him.