I just finished the game, and the snake (sea monster) was the hardest part IMO. Here's how I did it.
Walk out of the tomb, and veer over to the left bell. The cinematic will start, showing the snake rise up out of the water. Skip it, and begin shooting the bell. About 50% of the time you'll distract the snake before he has a chance to hit you, and you can drop the claw, for a first hit.
Now, after that, begin running straight over to the right side bell, but pan the camera around so that you can see the snake when he begins an attack. Be prepared to jump/dodge, etc, then start wailing on the bell, get his attention, and drop the claw.
Now, run down the center walkway and get almost to the end, and begin shooting either of the bells to distract the snake. This part isn't too bad, you just have to keep an eye on the snake and predict when he's about to strike. He seems to only use his downward strike at this part, so dodging backwards is usually the key.
Just keep shooting the bells to distract him, then drop the claws. Just make sure to concentrate on one bell at a time.
If you have alot of assault rifle bullets, then use that, as they fire faster than the pistols and will "charge up" the bells faster. Be quick with the grapple on the lever! As soon as you see the snake run to the bell, stop shooting and hammer the X button until you let loose your grapple, then once it hits the lever, hammer on Y.
It took me around 30 deaths, maybe more to finally beat this guy. Probably didn't help that I didn't have any health kits either lol.