WCEMS is a excellent service. They hire twice per year, in January/February and August. The new hiring class starts the academy on Monday (I'm in it!). Watch the website for applications to open up for the August class.
As far as difficulty, the testing is tough. It starts with an online personality/work ethic test, which is no big deal. Then you show up for the written exam, which is tough. Wilco prides itself on medicine; you should have a thorough understanding of the pathophysiology behind medical problems we see in the field. Those who pass the written move on to scenario testing. Pass that, and you go to interviews.
Study up and come test in August!