What study guides to buy depends on your background, if you are strong in biology then get destroyer, if you are strong in chemistry, buy KBB and cliffs AP
While the more practice you get the better, you want to manage your time and develop a study plan that is the most efficient for you.
If you carefully went through kaplan bb, cliffs AP bio or Barrons AP bio, Destroyer, CDP, achiever and topscore, you should break 20, but carefully going through them is very time consuming.
I'd say buy KBB first to gage your level, and then get study guides accordingly. (Topscore is a must)
Good luck