The problem is, there aren't enough flying units: dragons, tumblers, vampires, and djinns are too few to cause much trouble. In short, the catapult has so many hit points that it's not that easy to take it down.
Humans - griffins and angel
Demons - devils
Elves - fairies and dragons
High Elves - dragons
Mages - gargoyles, djinns
Necromancers - ghosts, vampires, heralds (if I'm not mistaken), dragons
Dwarves - dragons
