Air Stomp is hard to use, but it does cause some serious damage, let me say.
You have to make sure the time and place is perfect, because anything can change in a second. Fact is, this move is something you can't perfect in one go, you have to keep practising it until you know where and how to use it.
The best place to start an Air Stomp is on an elevated position, and of course, timing has to be perfect, otherwise it's not going to work.
You may not get it at first go, but take into account what I've said, then it shouldn't be a problem.