This Colossus is scared of fire, so you need to grapple and climb up one of the 4 pillars in the room. Once you are on top, the Colossus will ram the pillar, and drop a wooden stick. Grab this stick and hold it into the fire to ignite it.
You can now force the Colossus to back off and fall off the ledge and reveal its weak spot. You can jump from the ledge and hit its weak spot on the back. If you don't kill it from this, head back to the 4 pillars and jump off one of those to get on his back.