Force the Colossus to stomp you at either end of the arena where there is a ledge. This will cause the platform you are on to lift up and you can jump up. Now you are higher up, shoot your arrow at him whilst you are under the pillars. He will then attack with his sword and when he attacks a load of debris will fall down allowing you to jump up to the next section.
Now you are on the top, go to one of the bridges and stand on the edge of that. The Colossus will destroy it allowing you to take a leap of faith onto his head. His first weak spot is on his head.
Climb down onto his right arm and stab the weak spot on his tricep, he will then drop the sword that he is wielding. This allows you to climb down and reach his final weak spot, his palm.