I did this in just 3 save slots.
(Where you can unlock this - Right at the beginning when you get done with the hiding from Andrea segment.)
(Make sure to save after the segment too just in case something goes wrong, that way you can just load up the save and try again.)
What you need to do before doing this, make sure to upgrade ALL abilities (I made a guide to show you how to easily do that
) in the game so that you will be able to carry two defense items and you can just stab Andrea twice and she will go down.
Also make sure when you stab her twice to keep count of how many times you stab her. (that way you will know how many times you need to do this)
If you run out of defense items be sure to look at every drawer and cabinet, some are well hidden on boxes, in baskets, on top of drawers etc.
The easiest way to stealth stab her is going into the room where Jen and Lin had the argument, and turning on the TV in the room to make noise for Andrea to come in there, Make sure to go to the opposite side of the room (at the dresser with the mirror) in the corner and just wait for Andrea to approach the TV, you have a few seconds to sneak up and press A to stab her.
Repeat this process (I was able to get a total of 18 stabs on Andrea in 1 save)
If you can't find anymore Defense Items, make a new game on slot 2 (you will most likely need to make a 3rd new game slot too) and do the same things I told you to do.
Repeat the process until the achievement unlocks.