STAT is basically a glorified IQ test. There's no real need to prepare for it, nor do past papers offer you much advantage. The practice questions available on the ACER website give you a pretty good indicator as to what you'll be up against on the day.
I think there are some courses out there that offer preparation courses for the STAT, however, they are seriously a waste of money. As long as you aren't a total moron, passing it should be no issue.
I'm not entirely sure why Pharmacy uses it to be honest. From what I can tell, it's basically a way to ensure that prospective applicants are really keen on actually doing Pharmacy (to the extent that they'll book, pay for, and sit another test), as opposed to just putting it on the application and seeing if their ATAR is good enough.