Here are a couple of links to Adobe's certification process for Flex:
I also attached the PDF overview of the exam breakdown.
Free software for practice exams:
I got Flex 2 and 3 ACE certified by going though 3/5 of the Flex Builder help system TWICE for each exam, reading all the content and compiling all the sample programs. Do not read all the API docs in the help system, just the descriptive docs.
If this post answers your question or helps, please mark it as such.
Greg Lafrance - Flex 2 and 3 ACE certified
Flex Development, Training, and Support Services