The Giantslayers are a small sub-group of mercenaries within The Dozens. They consider themselves to be the "top dogs" of the faction.
When allied with The Dozens you may encounter them on south-eastern platform part of Woodend Plains next to the cart. You may deceive them, threaten them, or be honest that the job overwhelmed them. Asking to work together or being aggressive towards them will cause them to become hostile.
Assuming you don't kill them earlier, the Giantslayers' corpses will be found on the upper level of Lle a Rhemen, where they can be looted for an assortment of moderate loot.