Did you low the difficulty level?
I also thought it was impossible, anyway... I just beat the game
you need to be quick and organized; I destroyed all the ballist towers with fireballs, and each time I had Patriach spell available, I used it against a nest or a lightning tower (when it's not, I killed it with fireballs). Of course the wyvern is very useful, as well as dragon spirit (I used it each time I had less than 600hp). Move a lot to escape arrows, and don't mind about creatures (your wyvern is going to kill them). Don't forget any tower or nest, but be quick to be as ahead as possible from the zeppelin.
Well, it was my strategy and it was pretty useful. To be efficient, you may need some tries (to know where towers are,...)
