This fight is one of the easier ones. You're going to need to use your horse. The western cave has two entrances - you can choose any path you see fit.
You'll find yourself in a sandy cave. Your opponent is a snake-like lizard that swims in the sand. Use your horse and distance yourself from the opponent. When the colossus starts following you, equip your bow and shoot arrows at the boss. After a short while, the boss will open its eyes - you need to hit one of them. The colossus will hit one of the walls, giving you an opportunity to hit two vital areas on the body. You should be able to destroy a single vital area during a single knockdown.
Simultaneous dash, controlling the camera and shooting your bow may be difficult. Continue holding Triangle during the fight or the opponent may catch you until you've had an opportunity to shoot at his eyes.