This achievement can't be missed. All you have to do is beat this titanic-sized boss. Start by taking out the faces on each of his four legs. He will shoot missiles and laser bolts at you but they are easy to dodge. I recommend using the missiles to get some easy Witch Time. Periodically he will dive under the water and do two types of attacks. First, he will come at you from directly below so be sure to be quick on the dodge. Second, he will come at you from afar in a very Jaws-like fashion. This attack is much easier to avoid and you may not even need to use the dodge button if you're far enough out of the way.
After all four faces are gone you'll go into climax mode where you have to steer the boss into a giant spider. Occasionally he'll try to buck you off so be ready to hi Y+B to counter. You will do this a couple of times.
Once the legs are dealt with, his face will be exposed. Beat the crap out of it but beware his claw attacks. He will also try to bite you on occasion but he telegraphs it pretty well so dodge to avoid big damage.
After doing a decent amount of damage, he'll create a whirlpool (like a boss). Once you followi him down, he will blow you away so you have to make your way back to him while avoiding his projectile attacks. He has these tentacles that pop out and shoot giant lasers. They're pretty easy to avoid and are a good way to get Witch Time. Get in close to his face and slash away. Rinse and repeat a couple of times. Once his life bar is all the way down, make sure you are right in front of his face. I missed this on more than one occasion because I was caught up doing combos and he got a large chunk of life back. Once that is down you will be prompted to go into climax mode again for a sick finisher.
During my first playthrough I used the whip but then I switched to the claws. You have to do a lot of surfing around so make sure you're spamming guns (holding X) until you get close for a little bit of extra damage.
If all that text bored you to death, here's a more entertaining video from the guys at Achievement Hunter: