Bear in mind that you won't get the Achievement until the mission is complete and you have escaped the area - just picking it up isn't enough. I highly recommend spending the time picking off a few enemies before taking the mask as reinforcements will arrive. The safest way to the mask is to take out the enemies around the left, around the separate hut. There's a path above and behind the altar from which you can snipe, and the final few enemies will be facing away from you (towards the mask) allowing for takedowns.
Once you've taken it, remember you need to exit across the stone arch, NOT in the direction you entered by. So don't jump down anywhere. If you're quick at navigating, and you have a few health bars, you may well find the best strategy is to simply cheese it as fast as you can rather than attempting stealth or a straight fight. By the time you near the exit your enemies will have likely lost you.
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