There is an easier way to do this. The other solution takes about 90 minutes. But you can do this a lot faster.
I was exploring the world for:
Meanwhile the enemy collected 8 relics. Maybe you get a little bit less or more. But you don't have to waste your time to collect them. I have built a monastery in their area and collected all relics after nearly destroying everything. I just had one enemy villager left. And right before I killed him I saved the game and finished the map. Just load up your save for another win in a couple of seconds. Your collected relics will add up. It takes about 15 seconds to load up your save file. Took me about 2 minutes to get the achievement.