To unlock this achievement you will need to kill a zombie while you are in the air.
One of the easier ways I have found for doing this, short or just shooting and killing them, is to gather a group zombies behind you, run into the Wundersphere and throw a grenade at the area the zombies are at and use the Wundersphere. If you time this right, you should be in the air when the grenade goes off, thus netting you the achievement. Here is a video should you need some help:
Credit to iPrzemek for his/her video!