Obviously, you cannot even apply to take the exam without a manager vouching for your experience. But, making sure you have had ample hands-on experience in the direct care of critically ill patients will make it easier to understand complex pathophysiological processes and associated nursing actions that are on the exam.
Additionally, you should also make sure to have some experience in a leadership role, such as being a (relief) charge nurse, educating nursing students, orienting new nurses to the unit, or joining committees or other organizations in the hospital. This type of experience can be particularly helpful for the 20% of questions that focus on professional caring and ethical practice in critical care settings.