This should come naturally a little ways into the Siberian Wilderness. After the first encounter with the bear, you will need to make poison arrows at camp. Afterwards you will go to sleep, only to be awakened by gunfire. After a cut-scene you will encounter some human enemies. After taking the first one out from a tree perch, you should move forward into the bushes ahead as another approaches. This enemy will walk towards your location while on the radio with another person, simply perform a stealth kill before the conversation is over to unlock the achievement.