This colossus can be found in the ruins in the desert. The boss will appear behind your back after you land in the arena. Start running forward and climbing through the walls until you've reached the other end of the arena.
Hide below the temple and shoot at your opponent to get its attention. It will bow down - use the opportunity to grab a hold of its beard. Climb to the top of its head where you'll find the first vital area.
The second area is found on the "left kidney". As previously, it is a dangerous area, as you won't be able to rest while standing in its vicinity. If you are low on strength, return to the shoulder blade area, where you can safely stand and rest.
The final vital area is found on the outside part of the boss' left hand. You can (with some difficulty) climb on it from the ground or from the body, but there is a better way. Simply hide below the temple again and wait for the boss to bend down - the hand in question is used by the boss to support itself. Simply jump on the hand and attack the weak spot - it won't take much damage.