Just did this in 5mins after finishing the game without ever using the Bolt-action rifle.
Mode: Chapter Replay Elite
Level: The Acropolis
Diff: Adventurer (so you can take a heap of damage)
Simply run to the first cutscene (via campfire if you need to get the bolt-action rifle), swap to the bolt-action rifle, Shoot as many guys in the head as you can until they all die (or you do), reload checkpoint and repeat. Should only take 4 or so reloads, depending on how good a shot you are :) Easy.