Sniper Elite: Resistance

Using tall grass to conceal yourself then creeping up on an enemy is a great way to get the drop on them for a swift knife kill. Non-lethal takedowns obviously won't count – you need to press for a proper melee kill. A knife through the brain, that sort of thing. Accumulating 100 of these might require a couple of mission replays, but it's straightforward enough.

Using tall grass to conceal yourself then creeping up on an enemy is a great way to get the drop on them for a swift knife kill. Non-lethal takedowns obviously won't count – you need to press for a proper melee kill. A knife through the brain, that sort of thing. Also, if they are unaware, it will also count towards Le Fantome (15G). Accumulating 100 of these might require a couple of mission replays, but it's straightforward enough.

If you are playing on lower difficulties, if enemies are close to each other, you can just go from one to the other and kill them all with a takedown.

A good method to get this out of the way is to create a save just before you kill an enemy(s) with a takedown. Once they are, just reload that save and keep repeating until the achievement has unlocked.

To check your progress, press then navigate to Service Record > Medals > General and look under the ‘Combat’ heading to see your medal progress.