This is a missable achievement whose opportunity is immediately after arriving in Touissant, essentially right between the the "Envoys, Wineboys" quest and "The Beast of Touissant" quest. When you first arrive in Touissant and the credits for the DLC have finished you are immediately thrust into a battle with a giant.
To get the achievement you need to shoot him in the eye with your crossbow. A success is an instant kill.
It's a little tricky because he's wearing a giant bucket as a helmet and most of his face is blocked. I also found his "name" floating by his head obscuring my view of his face a bit, depending on the angle I was facing him at. This is much easier to do with the Lightning Reflexes skill (the one that slows down time when you aim the crossbow) maxed out and enabled.
Just keep a good distance away and try to aim for the opening near the center of the face. I actually got it when he charged straight at me, with time slowed for the crossbow skill, giving me a chance to really aim at his eye in the center of the opening of the bucket.
If Golyat should die before you get the achievement (either because you shoot him to death elsewhere or your companions kill him off) just reload the auto-checkpoint before the battle.