Conan

This battle is a lot easier than it seems at first.

Initially, the giant squid will be slapping his tentacles down on the deck. Just position Conan behind the mast, as the tentacles never land there. After a tentacle slaps down, run out and chop it off.

Next the squid will wrap a tentacle around the mast and try to pull the ship over, so quickly hack at it until it lets go. You need to do this fairly quickly or it's game over.

After this, the tentacles will start to grab the crewmen running frantically around the ship. Hack at the tentacle to make it let go of the crewman. If you fail to reach them in time, they'll turn into the oily zombie things and you'll need to kill them - or just grab them and throw them overboard.

Now the squid will go after the mast again, so quickly go to town on it to make it let go.

Once you've done that, the squid will start doing a combination of slamming the deck with tentacles and grabbing crewmen at the same time. I recommend hanging out behind the mast again and just doing away with tentacles that are close to you. The oil men are easy to dispose of, but getting hit by a slamming tentacle inflicts A LOT of damage.

Note that if you get low on health, you can use mastered attacks on the oil men to regain some of your health.

Finally, the squid will climb up onto the ship. He'll still be using the slamming tentacle attacks, as well as trying to snap at you with his beak. In addition, he has a slime/acid/whatever spewing attack that sweeps from side to side. Again, just hang out behind the mast to be protected from all of these attacks. Keep an eye out for oil men, however. Wait for the spewing attack to pass by you, then run up and chop the squid in the face 2 or 3 times, then roll back and take cover behind the mast again. Keep repeating this strategy until you whittle his life bar down and get the series of QTE button presses to finally end the squids miserable life.

The achievement unlocks when the boss is defeated.

For those who prefers a visual guide.