You must first learn the Anatomic Knowledge Skill from the Catacomb of Sacred Waters optional Tomb. This tomb is located at the far East end of the Geothermal Valley with the entrance near the Aqueduct Ruins campsite. Solve the tomb puzzle and open the Codice for this reward. Now you just need to find deer and while aiming you should see their heart glow gold. Kill 15 deer with this and the achievement is yours.
*NOTE: I recommend getting the Double, Triple, and True Shot - Hunter Skills. The True Shot skill makes a locked on target, a heart shot for animals and a head shot for people. The other skills are prerequisites for True Shot so you must have all of them first. Just get close enough to lock on and shoot it. Drop 15 with heart shots and the achievement unlocks.
After the story I just continued and fast traveled between the Siberian Wilderness, Soviet Installation, and Geothermal Valley to "repopulate" the area with deer to shoot.