A few tidbits of info I think should be in the guide:
-Unlike the first game, you CANNOT back track at all. Once you leave a level, there is no returning to it. I didn't realize this until like the 3rd level.
-On the Pause menu (accessed by pressing Start) there is a display showing you the status of Little Sisters in the current level. This is almost exactly like the first game's Big Daddy meter. The number of circular symbols indicates how many Little Sisters are in the current level. If the symbol shows a Big Daddy and a Little Sister, it means you have no adopted / harvested that Little Sister. The symbol changes to just a Little Sister face when you have harvested / adopted one.
-Big Sisters seem to consistently show up only after you have dealt with the Little Sisters in a given level.
-it is 100% possible to get all SP achievements in one go. However, because Brass Balls is kinda hard and does not require you to play the game on Hard, it seems like the easiest way to approach the game is like so:
--First Run: Play on Easy (or Normal if you think its boring), grab as many achievements as possible, including Brass Balls. Many of the SP achievements will temporarily skew your tactics, making them much more annoying to do on Hard. This also gives you a chance to really soak up the story and figure out the game mechanics and whatnot. Going for achievements like Savior requires that you take a hit on how much Adam you will be getting from the Little Sisters, which in turn makes the game harder, so again it makes more sense to handle that stuff on Easy.
--Second Run: Assuming you got your Brass Balls and all the random story Achievements handled, you can use your second run to knock out beating the game on hard, and also grab any of the easy-to-miss items like the audio diaries or weapon upgrade achievements.
Just my $.02. I just finished the game about half an hour ago. I would estimate playing on Easy and playing it at a "medium" pace (not going uber slow and looking carefully for every audio diary, and also not going as fast as humanly possible), it took me probably 15 hours to beat. This forum is helpful, as always, and with this game I have broken 47,000 on my Gamer Score