Fall to your death after the troll removes the bridge
South of the mirror room (see And beyond) is a hallway lined with coffins. You need to open the bottom right coffin for story reasons, but the one next to it contains a bag with copper coins. Open the bag and take the coins.
Break through the middle mirror and you will soon find yourself at a bridge with a troll who demands payment. Give him two of the coins and he will remove the bridge and throw you into the chasm, unlocking the trophy.