This area is only accessible after finishing off Moreau’s Reservoir, so make sure you’ve played up until that point in the story. Once you’re done there, take the crank you’ve recently acquired and use it to lower the drawbridge near the Ceremony Site. Once done, you can access a boat that’ll take you to all kinds of places.
Follow the river NORTH for a lot of goodies, but you’ll be able to disembark eventually on another jetty. Turn right after getting off the boat, and you’ll be inside a “treasure house”. \ Inside is an endless amount of moroaic and an eventual treasure.
All you’ve got to do is position yourself so that the lit brazier/torch is between yourself and the enemy and then shoot the torch. It’ll fling itself in the direction towards your enemy and get you the trophy or achievement.
And that’s pretty much it!
If you continue with this area, you’ll also uncover a decorative plate that can be sold to The Duke for a nice amount of lei. Who doesn’t love a bit of lei, honestly?