Petrificus Totalus is a sneak attack spell which can be cast while cloaked in Disillusionment. Simply cast Disillusionment on yourself and sneak up behind an enemy. You should get the option to press
and use Petrificus Totalus to incapacitate the target. Do this a total of 50 times to unlock the achievement.
Disillusionment is unlocked during the story mission "Secret of the Restricted Section" with Sebastian. You automatically get the ability to use Petrificus Totalus with it although not explained until later in the game.