Resistance 3

Replay 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 eachother. 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 ( ), 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 it too high, only 2 will puke, and too low, only 4. So make sure you practice the heighth a few times. 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. This will be easier to do if you have a level 3 mutator, as the poison cloud's radius is larger, but it can still be done with a level 2.

It can also be done using two shots from the secondary fire, as long as you fire the second as quickly as possible so that the barfing is still simultaneous. Shoot one at the group of four, then the second one at the group of two. This should make it easier if you are having trouble hitting all six with one. Alternatively, this can be done in the Cafeteria area in Graterford as well, as soon as you enter the area, launch the secondary fire to the far side of the room, through the door and you will easily hit 10+.

To see it in action, check out this video by sunnygrg: