When I saw the game I was going "WHOLY $%#(*&%@(*%*&@(*$@ THAT LOOKS SO COOL!" now that I have the game...I can't even get one kill with Air stomp
Now after this thread I think it's time for some crashsite.
Yeah man Crash-Site is your best bet. I was stuck at 10 AS Kills playing TM Instant Action and got a butt-load switching over to Crash-Site. I should've seen it earlier, but I didn't really dig Crash-Site previously. No I can happily say that I'm hooked.
As for not getting kills with it, be sure to have a lot of height to your advantage. There is no need to worry about taking damage from the fall because you don't take any when you activate the attack. And once you level it up to the second stage it rewards you with no fall damage permanently.
Good-Luck.