Chapter: Graterford, PA: Cellblock A
Right after you obtain the mutator and are released, you will kill 3 guards, and enter a room where your new friend will tell you about the mutator's secondary fire, and you will pick up 2 of them. Head in to the next area, where 4 men are on the ground floor talking to each other. Walk a little further and 2 more will come out of the door on the floor above them.
Now what you need to do is shoot your Mutator's secondary fire (R2), at the pillar holding up the second level. You need to shoot it so that it hits the middle of the pillar so the cloud infects all 6 of the Wardens. If you shoot the cloud and you don't get all six, then let them kill you and you can try again. After a few tries to perfect your aim, this trophy will unlock. Its best to use use the level 3 Mutator as I tried perfectly with a level 2 and didn't get the trophy.
Credit for video goes to sunnygrg on youtube.