To pass an Associate or Specialist level exam, we strongly recommend taking the corresponding course. See the main certification page for further details. Taking the training is no guarantee to pass, but few people pass without taking the training.
To pass a Professional level exam, it takes much more than training. The individual will need not only MuleSoft training but also significant hands-on project experience with MuleSoft products after the training. In some cases there are classes such as Anypoint Platform Development: Advanced to help increase knowledge, but again, the exam is meant to test real expertise, not just classroom knowledge.