In chapter 5, just after the helicopter crashes and you leave the old woman to investigate, you'll pass through a checkpoint on a cliffside where about 4 groups of birds will attack. After defeating the birds, instead of continuing to the crash site, walk across a log straddling the gap in the cliffs to your right. This log takes you back up to the area prior to the checkpoint. Continue back down the hill towards the crash, through the checkpoint again, and the same groups of birds should respawn and attack just like before. It seems that this loop can be repeated endlessly until the achievement unlocks. I was able to use this method to get the achievement without quitting out of the game or restarting.