Gol-Iath

The Gol-Iath is one of the Starting Ruins Colossal Titans you need to defeat and was featured in the game's demo. It's subtitle says Guardian of the first Gate

Tips for Beating
Shoot an Arrow from the entrance and draw it back. The fists will come from the left or the right depending on which side you're facing. You can run to the center of the room and slightly to the right, and be careful to dodge left (avoiding the first slam). When the non-active hand raises, shoot the target quickly. (Credit: youtube.com/user/Dacciox)

Alternatively, you can stand at the entrance and fire your arrow once you see his hand pull back. This will keep you from getting hit, since he cannot reach you.

A safe, albeit difficuly way is to walk into the stage only slightly, until the doorway is covering the first half of your body. Shoot straight up and roll outside the door before it closes, then draw back your arrow until it's roughly in the center of the stage. Run along the path towards the big door and stand only a little to the opposite side of the hand he's attacking you with to force his hands to switch. If timed correctly, you can draw back your arrow while the boss is switching hands, and kill him outside the arena. Keep in mind while you're running around the outside of the arena, his fists can reach you if you are close to the inside wall, so it is recommended to hug the outside wall all the way around for safe passage.

Video Guide
Tricky way to beat Gol-lath + Shadow of the Colossus achievement.Normal way to beat Gol-lath.