Track the Colossi
Players must travel to the location of a colossus before they can defeat it. The easiest way to do this is to travel to an area that is sunny and dismount your horse. You will be able to use your sword to focus sunlight beams in the direction of a colossus. Press the "Circle" button to hold your sword in the air, and move the cursor that appears by moving the joystick. Continue moving the cursor until all of the reflecting beams focus into one stream of light. The direction of the stream is where the colossus resides.
Recognize Patterns
Once a colossus has been located, it will attack you when you get close enough. All of the colossi have a limited set of moves that can be avoided if you can anticipate them. For example, the first colossus will attack you by swinging its club overhead and trying to smash you with it. Once you see the colossus bring its club overhead, you can dodge the attack by running to the left or right and pressing "R1" and "Triangle" together to roll out of the way.
Identify the Weak Spot
Each colossus has a weak spot that can be identified using your sword. Press "Circle" when you are close to a colossus to raise your sword. Move the cursor until the light reflecting off your sword streams in the direction of the colossus, which reveals its weak point. This process is similar to tracking a colossus. You must attack its weak spot to damage a colossus.
General Tips
Once a weak spot has been identified, you will often be required to climb the colossus to reach it. Colossi will often begin shaking when you climb them in an attempt to get you off. Hold onto the colossi when they shake by pressing and holding "R1" until they stop shaking. If you are damaged during battle, flee from the colossus until you are far enough away so that they do not follow you. Your health will slowly regenerate if you stand still. Continue battle only after your health has fully recovered.