Well, i just love to pwn PvP with fire(this is a great spec for air fighting:P)
Just check out this one.
spec You can have nice kills on the ones hovering over with flying mount like this:
Go above him(target range or more) dismount, slow fall, arcane power, presence of mind, (any other trinket buff or item), pyro, fireblast, cone of cold, and icelance if he's still alive :) With this spec i usually kill them in one run.
Btw, if you're expected down, just hit invis after finishing the job and go for cover and mount back up :)