Quit stressing! The last week you should be making a test day routine: get to bed on time, eat right, locate your test center, etc. Take a day off and watch a movie or something to relax. Then you can maybe do some light review of stuff that needs to stay fresh, like math equations, solubility rules, or stuff you tend to forget.
Have you at least taken a computerized practice test? If so, I don't think you would gain much by taking Achiever in the last week. Just my opinion.