I just took the HESI at Darton on the 26 of May. I did a heck of a lot better than I thought I would, and I didn't buy the study guide. I just made up my own flashcards based on what previous test takers said was on the exam. I scored a 90% overall, with AP being the lowest (80%, but still passing). It's all very straight-forward:
Know this: For AP, study the hormones, B cells, T cells, and the functions of cell bodies (ribosomes, endoplasmic reticulum, the membranes), functions of thyroid, thymus, pituitary. Then there are the basic physical anatomy questions like 'where is the calcaneous located?' and 'between what two structures is the mitral valve located?' (actual test question for me).
For Math: conversions - know how many milliliters in a gallon, how many ounces in a cup, etc. I found some good online sites and just made some flash cards for those. Also know how to convert pounds to kiligrams and vice-versa. Ratios and proportions! Add and subtract fractions! Change percentages into decimals into fractions, etc. You can use the basic calculator on the computer to help.
The vocab and reading parts were easy for me, scoring 100% in both. Just know how to spell and take your time reading so you can comprehend whats being asked.