Defeat 1st Colossus
Simply leap onto the back of the left leg and stab the calf. The colossus will kneel, allowing you to climb up the leg, then up the back and onto the head, where you’ll find the weakness.
Defeat 2nd Colossus
Get behind the colossus. Shoot an arrow at the bottom of one of the feet as it walks. Climb up the rear leg when the beast kneels, then head for the weak spots on its head and rear.
Defeat 3rd Colossus
Lead the colossus around the arena until you can get it to smash its sword into the central stone platform. The shock will break the armor of the colossus. Now wait for the colossus to swing and hit dirt, then climb up the sword, up the arm, and onto the head. Stab this weak point until it disappears, then look for the final weak point on the stomach.
Defeat 4th Colossus
Run to the furthest small hill and coax the colossus with arrows. Once it gets near, hop down and into the cave. Simply wait here while it stomps around. Eventually it will lower its head to investigate, and you can leap out of the cave and grab onto its - for lack of a better word - earrings. From here you can climb onto the top of its head and finish the beast off.
Defeat 5th Colossus
Swim to the middle of the lake and hop onto one of the small square platforms. Fire an arrow at the colossus and it will fly straight for you. Simply jump and hold the grab button to hitch a ride. Its weak points are at the ends of its wings and the tip of its tail. Should you fall, look for another small square platform and repeat.