Lies of P

Depending on your controller setup, it might be a different button. However, the preset is the RB .
You can sneak up behind a puppet that is staggering without being seen, and as you get close, there will be glowing slashes visible on the puppet's back. Holding the will unleash a Fatal Attack. If done successfully, the achievement will pop.
I did this easily by going right then upstairs after first entering the Krat Central Station and following the path to the left. I found a staggered puppet and followed the prompts to do a Fatal Attack.

You can also stagger a large enemy by holding the right trigger to charge an attack until the energy bar on the enemy flashes white, then while facing the enemy, press the RB.

Depending on your controller setup, it might be a different button. However, the preset is the RB .
You can sneak up behind a puppet that is staggering without being seen, and as you get close, there will be glowing slashes visible on the puppet's back. Holding the will unleash a Fatal Attack. If done successfully, the achievement will pop.
I did this easily by going right then upstairs after first entering the Krat Central Station and following the path to the left. I found a staggered puppet and followed the prompts to do a Fatal Attack.

You can also stagger a large enemy by holding the right trigger to charge an attack until the energy bar on the enemy flashes white, then while facing the enemy, press the RB.

El "Fatal Blow" es un movimiento que se puede ejecutar de 3 formas;

1° De manera sigilosa: La forma de efectuar esta manera es posicionarnos en la espalda del enemigo y cuando salga la figura del "Fatal Blow" ( Presiona ataque rapido, el cual si estas en configuracion predeterminada, el boton de esta accion seria )

2° Durante el combate: Lo que se debe hacer es posicionarse en la espalda del enemigo durante el combate, hasta que salga la figura del "Fatal Blow" seguido de un ataque rápido ( Presionando el botón de acuerdo a la manera predeterminada )

3° Cansando al enemigo: Durante el juego habrá jefes y mini jefes, que si cansas durante el combate, saldrá la opción de efectuar un "Fatal Blow" te darás cuenta porque su posición pasara de "ataque" a una con la guardia baja, en ese momento acércate hasta que salga la figura del "Fatal Blow" y efectúa un ataque ligero ( Presionando el botón de acuerdo a la manera predeterminada )


The "Fatal Blow" is a move that can be performed in 3 ways;

1° Stealthily: The way to do this is to position ourselves behind the enemy and when the "Fatal Blow" figure appears (Press quick attack, which if you are in default configuration, the button for this action would be [cn_RB ] )

2° During combat: What you must do is position yourself on the enemy's back during combat, until the "Fatal Blow" figure appears followed by a quick attack (Pressing the button according to the default way )

3° Tiring the enemy: During the game there will be bosses and mini-bosses, and if you get tired during combat, the option to perform a "Fatal Blow" will appear. You will notice because their position will go from "attack" to one with their guard down, At that moment, approach until the "Fatal Blow" figure appears and perform a light attack (Pressing the button according to the default way )

This achievement is probably going to be your first achievement, achieved at early Krat Station.
You have get behind a puppet for the Fatal Attack prompt (red marks on the back) and hit him (tap the light attack button, no need to hold) with heavy damage. P will show some flurry of attacks depending on the weapon equipped.

Timestamp: 00:06:10 (part 1 video)

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