The opportunity to do this comes right after completing the puzzle using all 6 of the chess pieces to unlock a new door. Since this is highly missable, it is recommend that you make a manual save in case you miss it.
Proceeding through the new doors will start this boss fight. During phase 1, run around the room avoiding the fire on the ground, and dodging the ceiling attacks. Once G enters the room, leave out the door he came through, moving onto the crane platform. Move the crane to start the fight. At this point, damage G, and down him at least 2 times (on assisted) or 4 times (on normal), to the point that his eye is dark red and at a certain point Leon will say "Bet I can use the crane", and then down him somewhere near the middle of the platform.
Once this happens, quickly interact with the crane to finish the fight using only one crane use.