Resistance 3
59 Achievements
20-25h
Backstabber
Kill 20 enemies with melée while they are idle
3.7%
How to unlock the Backstabber achievement in Resistance 3 - Definitive Guide
I take no credit for the video or strategy.
This is another picky trophy. In order for the backstab to be counted, the enemy needs to be standing completely still. No movement whatsoever. There are several opportunities for backstabs throughout the game, and your progress towards this trophy carries over across multiple playthroughs, and chapter select, so don't worry if you don't have 20 by the time the game finishes.
The easiest and fastest way to get this trophy is to farm it at a checkpoint. I found Graterford, PA: Cellblock A to be the best spot. After getting the Mutator, your cell will open, and a guard will be standing in the center of the block with his back turned to you. Simply walk up to him and melee ( ) him in the back. Now head over to the stairs and allow the other guard to kill you. Repeat the process until you have 20.
Another location:
I myself have found that the level Graterford, Cellblock B is better for idle kills than Cellblock A. In Cellblock B there are 5 guards who you can kill from behind, two on your left and three on your right as soon as you start that chapter. Use the Auger to see exactly where they are. But it appears the two standing guards going the toilet do not count as 'idle.' Once you have killed all five let the guards at the bottom next to the pool table kill you and keep continuing over and over until you get the trophy.
Like all other combat trophies, your progress can be monitored through the Extras menu, under the Rewards section.
Graterford Prison, Prison Building A - we leave our cell, kill the guard, let the second one kill us. We repeat the process until we get the trophy.
Graterford Prison, Prison Building B - There are 5 enemies here that can be killed quietly (without going down, 2 on the left, 3 on the right - we use the broken passages in the walls to get around them from behind), but those who do their business in the toilet do not count (probably fighting with their body ). After dealing with the enemies, let the enemies below kill you and repeat the process until you receive the trophy.
Squat and press CUT OFF YOUR LEG! Repeat 20 times. Most likely, this is how you should get the trophy (by cutting off your leg).