There is an even easier way to unlock this then how Shade (above) says to get it. You can get this achievement by saving only 15 survivors and getting ending B- you will only need to complete up to Case 2-3. Do the Cases as normal, but when you get the medicine in Case 2-3 (after killing Steven) DO NOT EVER TALK TO JESSE, this will give Brad the medicine which you don't want to do. By just holding on to it you prevent Brad from leaving and Barnaby from turning. This will give you Otis,Jesse,Brad,Barnaby and Frank making 5 survivors.
Now you will only need to save 15 survivors. This really isn't too hard and you can save whoever you find easiest. Obviously saving the 2 people on the roof is a smart idea. This means only 13 to go. I suggest saving as many people as quick as possible. The first day as few zombies, so its fairly easy to rescue people. On the second day the cult members come making it a little harder and on day 3 there are A LOT of zombies making saving people much harder.
Make sure you kill Adam the clown at 9pm, day 1. This will allow you to get another survivor and open up the Paradise Plaza-Wonderland shortcut which makes rescuing survivors much easier.
I won't go into much detail on who to save but I suggest as many survivors from Wonderland and Paradise Plaza as possible. And don't save Ronald the guy in Jill's sandwhiches unless you wan't to be forced to go back to the safe room later with food for him. If you don't many survivors will be lost and you'll have to replace them.
When traveling with survivors the A.I. can be a huge pain so use the way-point marker and clear the path in between it and the survivors your escorting.
Without much effort I was able to save 28 people so this achievement is definitely do-able.