Quote (gwokdude @ Sep 19 2010 10:50pm)
im not landing on his head i aim behind him and swing and ive gotten the animation but no achievement
Well I had to get the animation a few times before I got the achievement. The only reasons you would survive the fall (so you wouldn't get the achievement) are if sprint is still active or if you were going to hit him on the way down (not even landing directly on his head, if you touch him at all it could screw it up).