I've seen many people talking about how they died in mid air. But I've never seen anyone post it. (Not that I know of) Sometimes it will just happen randomly. But there is a way to do it on purpose. Here's how to do it on purpose. Its a little hard to do and its all about timing.
Method 1 - Get an npc to attack you and stand on the edge of a cliff or some other high area. Pay attention to the average amount of damage the npc is giving you. When you think your one hit from death run off the cliff. The npc will give you the last hit and you'll die in mid air.
Method 2 - Okay, I've never really tried this method but it might work if you try it. If it doesn't work just tell me and I'll edit this post. Get an npc to attack you. Pay attention to the average amount of damage the npc is giving you. When you think your one hit from death jump and the npc will give you the last hit.
I'm not sure if these will work all the time but when I did the first one it worked.