I think the fix can be accomplished in a less obtuse manner... Make the zeppelin nearly invulnerable with better offensive damage. On lower difficultly the zeppelin should be doing most of the work for you; it should be almost like an interactive cutscene. To improve gaming experience, the cooldown on "Eye of Patriarch" should be reduced to 1/2 or 1/3 so casual players can enjoy the thrill of taking out defenses in one swoop. Cooldown and zeppelin (health and damage contributions) may scale with difficulty to provide additional challenge.
I rather like that idea: sounds very cinematic and all you have to do is not get yourself killed. I'd have probably enjoyed that last stage if I hadn't spent it all worrying about the Zeppelin's imminent demise...
