Instructions
Find a clear vantage point of the arena below and either whistle or shoot an arrow at the Colossus to get its attention.
Run around to the other side of the floor as the Colossus climbs to the place where you whistled or attacked it. Find a clear vantage point and wait for the Colossus to climb up to your previous location.
Shoot arrows at one of its feet to briefly stun it and break that foot's hold on the wall. Fire again at a different foot to make the Colossus lose its grip and fall to the bottom of the arena. Climb or jump down after it as quickly as possible.
Climb up the Colossus' tail and onto its belly. Stab its glowing weak points as much as you can before it recovers from the fall. Get off of its stomach once it begins to stir and climb up and away from it as quickly as possible to avoid being hit. Find cover if it uses its energy attack and avoid the places that the attack struck for at least 10 seconds to avoid taking damage.
Repeat the process and make it fall again. Attack the glowing weak points until both are dispelled and the Colossus is defeated.