Easiest way to get this is later in the game after you defeat Moreau and get the crank, you can(kinda) head back to the castle to get the treasure that is shown on the map. You do this by using the crank on the drawbridge and taking the boat that is there towards the castle. Follow the path into the castle and you will see two unlit braziers on the left and right with a hanging one that is lit. Shoot the hanging brazier at the unlit ones to light on which will open their respective doors. From behind one of the doors enemies will start spawning and the goal here is to light them on fire and have them follow you to the last unlit brazier which is straight ahead from the entrance. Line up the shot and have it hit the enemy and boom there is your trophy. Have it follow you to light the last one and you have access to the treasure in this area.