Unsheath 5 swords from Immortal Skeletaurs without leaving the area

During the mission "Denial, Anger, Initiative", you will come to a battle arena where you must defeat 4 ghost kings. Amongst the 4 ghost kings are Immortal Skeletaurs. They are the ones with a shining blue sword sticking through their body.

Without leaving the battle arena, kill the Immortal Skeletaur and remove the sword from it's back. You must remove it rather fast because if not the skeletaur will level up. Do this 5 times and you will unlock the achievement.

You can come back at after completing the mission and attempt to remove 5 swords from the Immortal Skeletaurs