This achievemenment is actually really easy to get and with the right skills doesn't even require you to aim at the heart. First you just need the following skills...
- Anatomical Knowledge (Hunter skill tree) - Is automatically received after completing the Catacomb of Sacred Knowledge optional tomb located in the Geothermal Valley.
Now this is technically all you need to find the heart, but if you want an automatic heart shot every time you find a deer get these 3 next perks as well.
- Double Shot (Hunter skill tree) - First one in the second row.
- Triple Shot (Hunter skill tree) - First one in the third row.
- True Shot (Hunter skill tree) - Second one in the third row.
Now the two you really want is the one from the optional Tomb and True Shot (but to get True Shot you first need Double and Triple Shot).
Now that you have those skills just find some deer, best places to find some are in the Siberian Wilderness, Soviet Installation, and Geothermal Valley. Now once you find a deer just draw your arrow with
now you just need to hover over the deers body until you see a RED DIAMOND appear (which registers as a automatic headshot on humanoid enemies, but a heartshot on deer. Then fire, this will automatically give you a heart shot even if the deer is running away from you. 15 kills and the cheevo is yours.