After arriving in Toussaint during "The Best of Toussaint" quest, you will almost immediately run into your first boss of the expansion, Golyat. This giant has a wooden barrel over his head/face for protection and a large hammer-like weapon that he swings violently around the field. Treat him as you would any other large foe: dodge and roll away from his attacks, which are much like that of a Golem/Elemental.
Once you can position yourself at a distance, you will need to use your crossbow to score a shot at the small opening in his wooden helmet. If aimed correctly it will instantly kill the giant, regardless of how much health he has remaining. While not necessary, the "Lightening Reflexes" skill in the Combat tree may help in this setup as it slows down time by 45% when aiming the crossbow.
If Golyat happens to die from your allies or by any other means, be sure to reload your latest autosave to try again.