from the main menu.
Once it loads, head through the gate directly in front of you into an open area. Once through the gate, go forward about 3-4 steps and look to the right to see a cave opening with cobwebs. Walk towards the cave and you will get attacked by a creature, which will show a button prompt QTE on your screen. Press the button in order to kill the creature, which will gain your Koins and also unlock the achievement.