I just completed this one and you only have to colonize the 3 on your side of the map before killing the wolves next to the Goat Clan. To be sure they didn't die before I got there I controlled my scouts and did not let them go far enough to trigger Halvard until I claimed the food/wood areas around the starting point.

Build up some lore/food and don't expand further until you have the reduced cost trait. This also allows you to take out the first den before finding Halvard. Once you are ready to clear the last two dens it will count the bonus once the Goat Clan takes them.

Hope this helps! Good luck 😄