Talk to Kaan (the Gnomes’ chieftain) on The Isle of Thieves, and he’ll give you this quest. This boss fight is very difficult (I found it to be by far the hardest in the game), especially since the creature will eat any weapons you have equipped except the Titan Spear. The best way of doing it, then, is to climb up to the back of a tiny little ledge on the right side of the clearing (it has what appears to be the head of a fallen statue lying upside down in front of it), and keep throwing the spear at the monster. If you’re positioned correctly near the back, it shouldn’t be able to hit you, even if it climbs up the wall itself. The parrot is also apparently useful here, though I didn’t use it myself.