In order to make someone kill one of their allies you must first learn Possession (Level 2), which will cost a total of 8 runes, as well as Bend Time (Level 2), which will cost you 10 runes. Due to their high rune costs you will not be able to learn them both until Mission 3: House of Pleasure at the earliest—that is if you find every rune up until that point.
The most reliable method is to have a soldier shoot one of their allies with a pistol, and is the same method shown in the Harm’s Way guide. Mission 9: The Light at the End is especially good for doing this—which is the location shown in the video—because every soldier has a pistol. The first step is to locate two more soldiers in the mission.
About two seconds after an enemy points their pistol at you they will fire. It is the same delay for each and every type of enemy. The cast time on Bend Time is also about one second. For a kill to count, simply use Bend Time (level 2) a second after one of them fully extends their pistol and aims it at you, or reloads after firing. The moment bend time activates the target should have just fired their gun.
Immediately use possession (level 2) on the soldier that did NOT shoot the pistol; if you possess the soldier that fired it will count as a suicide when they die to the bullet. Walk the possessed soldier in front of the bullet (be sure not to walk into it), and position him a few feet away from it—not right in front of it otherwise it will miss—then release the possession. It will instantly kill them and count towards the trophy. If only one soldier remains immediately flee and find an additional one.
It is best to have at least five or six mana and health potions before attempting this and to save first. You should also save right after you perform a successful kill. Load up the save if you make a mistake and repeat the process until it unlocks.
To see a walkthrough of the the method being performed, please see the video below:
This trophy can also come naturally during your high chaos playthrough. Occasionally, if a guard tries to shoot you but there is another guard in between you and the shooter, his bullet will hit the other guard and can kill him. This is not a sure-fire way of unlocking this trophy, but it is possible as I earned the trophy this way.
No comments? Well for the young'uns. The easiest way to do this is to simply play thru the game killing every enemy u find. (i didnt harm anyone that wasnt a soldier). There are many places in the game where u will be attacked in tight corridors. its only a matter of time before someone pulls the trigger while someone is in the way. Dont hide from fights if u are looking to finish in high chaos. Feed it to them!! Its just a matter of time b4 they shoot their allies accidently.
In the level where slackjaw asks to see you, knock out 5 of his thugs. Lay them out on the ground in a tight pile. Attack another thug so all the other thugs start chasing after you. Play ring around the rosie with those thugs around the unconscious thug pile. The attacking thugs will use their fire breath attack to try to get you, inadvertently killing the unconscious thugs on the ground.
This popped for me during my Dark and "Mostly Flesh and Steel" playthrough on mission 8. So I didn't even need Possession. I accidentally alerted some guards in a small room with one entrance and I was mowing them down by the doorway when they rushed in and were slashing and shooting at each others back, thus killing their own guys too... geniuses lol So that worked for me