I'm having trouble unlocking this achievement, at first I thought it was just me. I've recently discovered 2 other people that have a similar problem. This achievement will not unlock but instead slowly progress up a little every time you successfully levitate up 50 blocks from the attacks of a Shulker. I've done this over 20 times and still have not unlocked it, others have had to do it 10 times, so it varies I believe (unless I can't count). It does not affect everyone.
So yes there is a solution to unlocking it but it's definitely not fun or quick and easy.