Even the best specialists from any field can face difficult clients and there is a variety of reasons behind that. Sometimes the performer himself can mess up with the work, sometimes it's actually a tough client, and sometimes it's a simple misunderstanding between the two parties. Whatever the reason is, it is necessary to avoid such scenarios.
Do your job properly!
Perhaps the most effective way to avoid a conflict situation is to do your job as perfectly as possible. If you do a good job the chance that you will face any problem significantly decreases. I don't think that we need to discuss this suggestion any further since each specialist should already have it in his head by default
Contracts and Briefs
Web-studio, freelancer, a group of freelancers, or whoever you are, you should always work according to a clearly structured brief and contract that sets out all of the project details, payment terms, and obligations of each party. The more details you discuss before you started working on a project, the easier it will be for you. Indeed, filling out a contract is not too fun, but it is much better to spend a little bit of time in the beginning and save your priceless nerves.
Customer's criticism
It is important to have the right approach to criticism. Remember - CRITICISM IS NOT A BAD THING! On the contrary, for you, this is an occasion to find the disadvantages within your service or product, understand the problem and fix it. It's like getting a free audit of your service. With each new criticism, you will become better and better and at some point, customers will not be able to find what to criticism, because you will be providing the best product and service due to the right approach to criticism.