During the mission "Denial, Anger, Initiative", you will come to a battle arena with a giant tree in the middle of it. In order to progress the story you must defeat 4 ghost kings. Some of the lesser enemies that spawn during the first half of the fight are the Immortal Skeletaurs.
Once you kill one, walk up to it and "unsheath" the sword sticking through its body. There will be a prompt to press and pull it out. They will keep coming back to life until you pull the sword from their corpse (hence the Immortal part of their name). Any Immortal Skeletaurs killed on this map will count toward the total so long as you haven't travelled since killing them. I killed 2 prior to getting to the arena and the achievement unlocked upon pulling the sword from my 3rd Skeletaur during the fight. If playing co-op any swords pulled by any member of your party will count toward the total and you will all get the achievement.
Note: If you failed to get this by the end of the campaign, you can go back to the same arena later and redo the fight to find enough of these guys in the same area.