This is earnt in Yamantau level.
It's pretty simple and there's a bonus towards this if you're on the hardest difficulty.
You'll die ALOT in this level if you're on Ranger Hardcore (like myself) the good thing is that the game remembers how many Cannibals have died during the level so, although it can get annoying, this will unlock automatically.
If you're on an easier difficulty and want to make sure you get this, there's a great little boost spot.
Eventually you'll hear Sam over the radio say that he's pinned down as there's loads of Cannibals all over his level. Infront of Artyom is five Cannibals taking aim at Sam. Reisit the urge to steam in and slap them all silly and carefully crouchwalk around the corner and aim for the gas cannister on the floor by the scaffolding with a single man on it. All five will die. Two flamer enemies spawn next kill them with your rifle. Now the guy on the bridge. Throw a few Molotovs through the open door and four more enemies will bite the dust. Run in to the next corridor and die on purpose. Reload the checkpoint, rinse and repeat until it unlocks.