Wolfenstein: The Old Blood

To unlock this achievement, you will have to perform 25 enemies using stealth kill takedowns.

The end of Chapter 1 - Prison Break allows you to quickly complete 25 stealth takedowns of enemies from behind. As you exit out of the vents, crouch (stealth) as you approach the first commander of the game. Use the Takedown (default is Mouse 3) command to instantly kill the enemy when prompted. Now select "reload from last checkpoint" from the menu and rinse, wash, repeat until you have 25 stealth takedowns.

There's many opportunities to get this, but an easy way would be in Chapter 7 after you get into the mech.

As you proceed through this chapter on the mech, you will see that lots of zombies will spawn, so all you have to do is equip a weapon while you're on the mech and throw a grenade when you have enough zombies gathered. Zombies won't try and run out of the way when you throw a grenade at them, so it makes it one of the easiest ways to get this perk and achievement.