DOOM 3: BFG Edition
59 Achievements
50-60h
Turncoat
Get 2 demons to fight each other in DOOM 3, RoE, or Lost Mission
11.2%
How to unlock the Turncoat achievement in DOOM 3: BFG Edition - Definitive Guide
The video below shows where exactly you get ambushed and how to get it. The video isn't mine and I take no credit for it.
After the elevator rises, avoid the zombie, soldier and run to room exit in the right. Outside in the left will be a soldier near a barrel. Run into his position to lure 2 Cacodemons near the Car running. Return to the room and every monster (Imp, Zombie, Soldier, Cacodemon) will try to hit you but without space for it and the game will have a huge slowdown. Dodge a little and the achievement is yours after the Cacodemons tries to hit you but the wall of enemies in front of your character will not let it happen. You will probably die even in the Recruit difficulty level if not using the Plasma Gun.
In DOOM 3 you can get demons to fight each other. This is done by a less powerful enemy accidentally attacking a more powerful enemy. You may get this without even trying or you might not. What you have to do is get, let's say an imp and an arch demon in the same room. If you can line it up so the imp will throw a fireball towards you but then it ends up hitting the arch demon the two will fight, with the Arch Demon winning. There's two good spots for this:
Hell (DOOM 3)
After the fight with the Zombies and Lost Souls you'll enter a room, opposite that room is a cliff gave and behind a rock is the BFG. Run into that area and just sit there, make sure you saved prior to this part. There will be two arch demons that can't fit through the gate (Don't kill them), plus some imps will be on the other side. It won't work every time but just keep pacing back and forward and an imp will most likely hit an Arch Demon. This is a very good spot as the Arch Demon killed two Imps when I did it. You had best do this on Recruit.
Hell (Lost Mission)
Near the end you are going through a sort of castle complex, what you want to do is just keep running around and running towards the next area. They will be tons of demons of all types here and chances are one will hit another. If this trophy doesn't unlock by the time you reach the last boss, reload your save and try again.
This is one gameplay mechanic of Doom that I have always loved. Monster infighting!
The way that infighting works in Doom 3 however is when a lower ranking demon hits a higher ranking one. So for instance, you will need to get an Imp to hit a Cacodemon, in which leads the Cacodemon to destroy the Imp. The Imp most likely will not fight back.
Thanks again to Minahjjang.