This is an easy achievement. You just need to Ghostwalk through 500 enemies.
Ghostwalking is just the evading technique for the Necromancer much like the Elf's roll technique. It can be used by pressing the shift button while the cross hair is in the direction you want to go or right analog stick in the direction you want to go. You do not need to kill the enemy with this move. Just need to go through them.
Using a controller is your best method of getting this achievement. It gives you much better control over the technique.