Just got it in multiplayer. Had a friend of mine helping me, he stayed with the skeleton so it wouldn't despawn, and I made a pillar far away. I could barely see the skeleton, but got the achievement nevertheless.
Is it possible to get the achievement if you are >50 blocks high? Or does it only count as 50 blocks horizontally? But I tried to get it a while ago, I was at the edge of the render distance, on quite varied ground, it wasn't flat, lots of trees around. Not great for getting that achievement I guess.
Also does it have to be 50 blocks exactly?
EDIT: WOOOP I just got it was on ground level on a superflat world