Organic Shield I found the easiest spot (this took my 2nd attempt, around 5 minutes after trying it) is on mission 5 "Expendable". Once you get at the bottom of the fire escape grab a zombie by holding
and start shooting the hell out of them, headshots if possible but not entirely needed.
After all the zombies are dead walk forward a little more and the door to the left will explode with 2 zombies on fire coming out, kill them for 2 if you need them. If you still don't have it, turn around and shoot those zombies. If you are playing on the easiest difficulty, the zombies will go down quickly and this achievement is a breeze.
There is no Crimson heads at this point in the mission. (Crimson heads are the fast red zombies that attack really fast).
Good luck and have fun.