This trophy is highly missable but it is really near the beginning of the game.
During the 2nd encounter with the basilisk (the place that prompt you a message to kick ladders down).
Instead of shooting the ballista at the basilisk, turn it so it aims at the building on the left of the basilisk(lets say about 45 degree you turn it), there's only 3 spot the ballista can be turned, main spot, to shoot the building or to shoot the basilisk.
When you'll shoot you will destroy the building and get your trophy