During the main story missions you'll come across an area were you have to defeat four resurrected Kings.
While doing this you will be attacked by skeletons, archers and most importantly Immortal Skeletaur´s.
For the achievement you must remove the sword from their back 5 times (not 5 times to a single one, but rather to 5 differente Skeletaurs).
They will keep contanstly spawning so this shoulnd't take long, I believe if you die you might have to re-do the section since when you die, you "technically" leave the area.
But!, you can come later with a high level character and just focus on the skeletaurs while ignoring the Kings.
For people that already got this on 360 and have an imported character. It just took me 1 Skeletaur to get the achievement.
Good luck!