This requires an intentional glitch of sorts, when you walk into the fight you'll want to fire your arrow all the way up and then roll back out the door you came from before it closes. Your arrow will now be stuck inside the room and you can move it around by pulling it. Try to get your arrow somewhat near the middle of the room them proceed up the stairs to the top. This is where you'll need to pull from but you'll need to time it right. As you move from the left to right side and vice versa he will attack with a different hand leaving his weak point exposed. Wait until he begins to attack and switch sides, after the fist has landed you can pull back your arrow and it should hit just as the Titan switches hands.