I have killed at least 4 skeletons 50 meters above them and yet no achievement what should I do
The achievement is actually 50 blocks away, horizontally.
You need to do it from 50 BLOCKS away for it to count, and your saying youve been doing it from above them? Chances are, either the game registers only from the ground, OR you simply may not have measured correctly from the top (Just so you remember, the mobs locations from you is based off the block the bottom half of their body is In, same goes for the player, so make sure not to base it off their head or the block their standing on.
Edit: I measured wrong originally, heres a better explanation ^